Simultaneous measurement of thyroxine and triiodothyronine peripheral turnover kinetics in man

Combined LT3 and LT4 therapy for precision medicine: easier with TTCombo system

Hypothyroidism is typically treated with levothyroxine monotherapy. However, despite normalized serum thyroid-stimulating hormone levels, 5-10% of patients continue to experience persistent symptoms, raising concerns about the adequacy of thyroxine monotherapy. Combination therapy with levothyroxine and liothyronine has been proposed as an alternative, but it presents practical challenges, including dosing complexity, the short half-life of triiodothyronine, increased monitoring requirements, and potential adverse effects. Moreover, there is no clear consensus within the medical community regarding the superiority of combination therapy over levothyroxine monotherapy, although some studies indicate potential benefits in specific patient populations. Genetic factors, such as polymorphisms in the DIO2 gene, may influence individual responses to therapy, further complicating treatment. To address the limitations of combination therapy, we propose a novel approach: TTCombo. This digital health technology delivers personalized doses of levothyroxine and liothyronine, improving treatment adherence and optimizing outcomes. By providing individualized, physiologically tailored hormone replacement, TTCombo has the potential to revolutionize hypothyroidism management and enhance patient quality of life.

A Randomized Clinical Trial Comparing 2 Levothyroxine Regimens During Ramadan Fasting in Thyroidectomized Patients

Context

For Muslim patients on levothyroxine (L-T4) therapy, the best approach for L-T4 intake during Ramadan fasting remains unclear.

Objective

We compared 2 practical approaches for L-T4 intake during Ramadan.

Methods

We randomly assigned 69 patients (21 males, 48 females, median age 44 years) with differentiated thyroid cancer (DTC) who underwent thyroidectomy in the past and are on stable LT4 doses to 2 arms. Arm A (33 patients) ingested their pre-Ramadan L-T4 dose at the evening meal and ate immediately. Arm B (36 patients) increased their pre-Ramadan dose by 25 µg if their regular L-T4 dose was ≤150 µg/day or by 50 µg if their pre-Ramadan dose was >150 µg/day and ate immediately.

Results

At the beginning of Ramadan (baseline), the median thyrotropin (TSH) level and the numbers of patients in euthyroidism, subclinical hyperthyroidism (Shyper), or subclinical hypothyroidism (Shypo) were comparable between the 2 arms (P = .69 and P = .65, respectively). At the end of Ramadan, in arm A there were 17 (51.5%), 3 (9.1%), and 13 (39.4%) patients in euthyroidism, Shyper, and Shypo compared with 17 (47.2%), 14 (38.9%), and 5 (13.9%) patients, respectively, in arm B (P = .005). The mean ± SD TSH levels in arms A and B at the end of Ramadan were 5.6 ± 6.0 mU/L and 1.67 ± 2.6 mU/L, respectively (P = .0001).

Conclusion

No overt thyroid dysfunction developed but there were more cases of Shypo in arm A and Shyper in arm B. Arm B achieved desirable levels of TSH (normal or slightly suppressed) in 86% of cases and might be a preferable approach, especially for patients who need TSH suppression (eg, DTC).

Perioperative growth hormone levels as an early predictor of new-onset secondary adrenal insufficiency following transsphenoidal pituitary tumor resection

OBJECTIVE

This study aims to predict new-onset secondary adrenal insufficiency (NOSAI) after transsphenoidal pituitary tumor resection surgery using perioperative growth hormone (GH) and prolactin (PRL) levels, among other factors.

METHODS

A cohort of 124 adult patients who underwent transsphenoidal resection for non-functioning pituitary adenoma, with routine perioperative glucocorticoid use, was used to develop the predictive regression model. An additional 46 patients served as the validation cohort. Generalized additive models were used to identify optimal cut-off points for the variables.

RESULTS

The GH level on postoperative day one (POD1) can be a simple predictor by implementing a cut-off point of 0.41 ng/ml. A value ≤ 0.41 ng/mL predicted NOSAI with 0.6316 sensitivity and 0.7810 specificity for the original cohort and 1.0000 sensitivity and 0.7143 specificity for the validation cohort. The multiple logistic regression model included perioperative PRL level difference, perioperative GH level difference, intraoperative cerebrospinal fluid (CSF) leakage, tumor size, and the combined effect of diabetes insipidus (DI) and relative perioperative GH level difference. The areas under the receiver operating characteristic curves were 0.9410 (original cohort) and 0.9494 (validation cohort) for the regression model.

CONCLUSION

Early morning GH level on POD1 can predict NOSAI with fair accuracy when perioperative stress dose glucocorticoid is administered. Prediction accuracy can be improved by considering CSF leakage, DI, and perioperative changes in GH and PRL in the final regression model.

Levothyroxine personalized treatment: is it still a dream?

Levothyroxine is a milestone in the treatment of all causes of hypothyroidism. From 19th century till today, Levothyroxine experienced a great advancement, from hypodermic injections of an extract of the thyroid gland of a sheep to novel formulations, known to circumvent malabsorption issue. However, the rate of patients on suboptimal therapy is still high. Current Guidelines are clear, daily Levothyroxine dosage should be calculated based on body weight. However, we are still far away from the possibility to administer the right dosage to the right patient, for several reasons. We retrace the history of treatment with levothyroxine, pointing out strengths and weaknesses of different formulations, with particular attention to what keeps us away from tailored therapy. In the age of digitalization, the pharmaceutical industry has been giving rising importance to Digital therapeutics, that are known to be effective in reaching target therapies. By combining current knowledge of hypothyroidism therapy with cutting-edge technology, we also hypothesized what could be the future strategies to be developed in this field.

The Use of Levothyroxine Absorption Tests in Clinical Practice

Although levothyroxine (LT4) is a widely prescribed drug, more than 30% of LT4-treated patients fail to achieve the recommended serum level of thyrotropin with a body weight-based dose of LT4. An LT4 absorption test (LT4AT) is part of the workup for confirming normal LT4 absorption or diagnosing malabsorption. We searched PubMed with the terms levothyrox*, L-T4, LT4, TT4, FT4, FT3, TT3, test, loading, uptake, absorp*, "absorb*, bioavailab*, bioequiv* malabsorb*, and pseudomalabsorb*. A total of 43 full-text publications were analyzed. The published procedures for LT4AT differ markedly in the test dose, formulation, test duration, frequency of blood collection, analyte (total thyroxine [TT4] or free thyroxine [FT4]), metric (absolute or relative peak or increment, or area under the curve) and the threshold for normal absorption. In a standardized LT4AT for routine use, the physician could advise the patient to not consume food, beverages, or medications the morning of the test; administer 1000 µg of LT4 in the patient's usual formulation as the test dose; ensure that the patient is supervised throughout the LT4AT; perform a 4-hour test, with hourly blood samples; assay FT4; and consider that normal LT4 absorption corresponds to an FT4 increment of more than 0.40 ng/dL (5.14 pmol/L) or a TT4 increment of more than 6 μg/dL (77.23 nmol/L) for a test dose of at least 300 µg, or a percentage TT4 absorption of more than 60%. If the test indicates abnormal LT4 absorption, the physician can increase the LT4 dose, change the formulation or administration route, and/or refer the patient to a gastroenterologist.

A minimal human physiologically based kinetic model of thyroid hormones and chemical disruption of plasma thyroid hormone binding proteins

The thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3), are under homeostatic control by the hypothalamic-pituitary-thyroid axis and plasma TH binding proteins (THBPs), including thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin (ALB). THBPs buffer free THs against transient perturbations and distribute THs to tissues. TH binding to THBPs can be perturbed by structurally similar endocrine-disrupting chemicals (EDCs), yet their impact on circulating THs and health risks are unclear. In the present study, we constructed a human physiologically based kinetic (PBK) model of THs and explored the potential effects of THBP-binding EDCs. The model describes the production, distribution, and metabolism of T4 and T3 in the Body Blood, Thyroid, Liver, and Rest-of-Body (RB) compartments, with explicit consideration of the reversible binding between plasma THs and THBPs. Rigorously parameterized based on literature data, the model recapitulates key quantitative TH kinetic characteristics, including free, THBP-bound, and total T4 and T3 concentrations, TH productions, distributions, metabolisms, clearance, and half-lives. Moreover, the model produces several novel findings. (1) The blood-tissue TH exchanges are fast and nearly at equilibrium especially for T4, providing intrinsic robustness against local metabolic perturbations. (2) Tissue influx is limiting for transient tissue uptake of THs when THBPs are present. (3) Continuous exposure to THBP-binding EDCs does not alter the steady-state levels of THs, while intermittent daily exposure to rapidly metabolized TBG-binding EDCs can cause much greater disruptions to plasma and tissue THs. In summary, the PBK model provides novel insights into TH kinetics and the homeostatic roles of THBPs against thyroid disrupting chemicals.

Intrathyroidal feedforward and feedback network regulating thyroid hormone synthesis and secretion

Thyroid hormones (THs), including T4 and T3, are produced and released by the thyroid gland under the stimulation of thyroid-stimulating hormone (TSH). The homeostasis of THs is regulated via the coordination of the hypothalamic-pituitary-thyroid axis, plasma binding proteins, and local metabolism in tissues. TH synthesis and secretion in the thyrocytes-containing thyroid follicles are exquisitely regulated by an elaborate molecular network comprising enzymes, transporters, signal transduction machineries, and transcription factors. In this article, we synthesized the relevant literature, organized and dissected the complex intrathyroidal regulatory network into structures amenable to functional interpretation and systems-level modeling. Multiple intertwined feedforward and feedback motifs were identified and described, centering around the transcriptional and posttranslational regulations involved in TH synthesis and secretion, including those underpinning the Wolff-Chaikoff and Plummer effects and thyroglobulin-mediated feedback regulation. A more thorough characterization of the intrathyroidal network from a systems biology perspective, including its topology, constituent network motifs, and nonlinear quantitative properties, can help us to better understand and predict the thyroidal dynamics in response to physiological signals, therapeutic interventions, and environmental disruptions.

Thyroid hormone regulation of adult hippocampal neurogenesis: Putative molecular and cellular mechanisms

Adult hippocampal neurogenesis is sensitive to perturbations in thyroid hormone signaling, with evidence supporting a key role for thyroid hormone and thyroid hormone receptors (TRs) in the regulation of postmitotic progenitor survival and neuronal differentiation. In this book chapter we summarize the current understanding of the effects of thyroid hormone signaling on adult hippocampal progenitor development, and also critically address the role of TRs in regulation of distinct aspects of stage-specific hippocampal progenitor progression. We highlight actions of thyroid hormone on thyroid hormone responsive target genes, and the implications for hippocampal progenitor regulation. Given the influence of thyroid hormone on both mitochondrial and lipid metabolism, we discuss a putative role for regulation of metabolism in the effects of thyroid hormone on adult hippocampal neurogenesis. Finally, we highlight specific ideas that require detailed experimental investigation, and the need for future studies to obtain a deeper mechanistic insight into the influence of thyroid hormone and TRs in the developmental progression of adult hippocampal progenitors.

Adjustment of the iodine ICRP population pharmacokinetic model for the use in thyroid cancer patients after thyroidectomy

Biokinetic models developed for healthy humans are not appropriate to describe biokinetics in thyroid cancer patients following thyroidectomy. The aim of this study was to adjust the population model for iodine proposed by the International Commission on Radiological Protection (ICRP) for the use in these patients. Rate constants of the ICRP publication 128 model for iodine were adjusted using the population modelling software package Monolix to describe activity retention in whole-body, thyroid, blood and protein-bound iodine observed in 23 patients. The new set of rate constants was compared to the four uptake scenarios proposed in ICRP publication 128. Flow from the inorganic iodide in blood compartment into the first thyroid compartment decreases to 0.15 d^-1compared to a value of 7.27 d^-1for the ICRP publication 128 model with a medium uptake. The transfer from first to second thyroid compartments and the outflow from the second thyroid compartment increases. An increased turnover rate of extrathyroidal organic iodine is observed. The rate constant from inorganic iodide in blood to kidney was also adjusted. Overall a good agreement was found between the adjusted model and the activity retention in thyroid cancer patients. The adjustment of population pharmacokinetic models to describe the biokinetic properties of specific patient populations for therapeutic radiopharmaceuticals is essential to capture the changes in biokinetics. The proposed set of rate constants for the established ICRP publication 128 model can be used to more accurately assess radiation protection requirements for the treatment of thyroid cancer patients using radioiodine.

Levothyroxine Treatment and the Risk of Cardiac Arrhythmias - Focus on the Patient Submitted to Thyroid Surgery

Levothyroxine (LT4) is used to treat frequently encountered endocrinopathies such as thyroid diseases. It is regularly used in clinical (overt) hypothyroidism cases and subclinical (latent) hypothyroidism cases in the last decade. Suppressive LT4 therapy is also part of the medical regimen used to manage thyroid malignancies after a thyroidectomy. LT4 treatment possesses dual effects: substituting new-onset thyroid hormone deficiency and suppressing the local and distant malignancy spreading in cancer. It is the practice to administer LT4 in less-than-high suppressive doses for growth control of thyroid nodules and goiter, even in patients with preserved thyroid function. Despite its approved safety for clinical use, LT4 can sometimes induce side-effects, more often recorded with patients under treatment with LT4 suppressive doses than in unintentionally LT4-overdosed patients. Cardiac arrhythmias and the deterioration of osteoporosis are the most frequently documented side-effects of LT4 therapy. It also lowers the threshold for the onset or aggravation of cardiac arrhythmias for patients with pre-existing heart diseases. To improve the quality of life in LT4-substituted patients, clinicians often prescribe higher doses of LT4 to reach low normal TSH levels to achieve cellular euthyroidism. In such circumstances, the risk of cardiac arrhythmias, particularly atrial fibrillation, increases, and the combined use of LT4 and triiodothyronine further complicates such risk. This review summarizes the relevant available data related to LT4 suppressive treatment and the associated risk of cardiac arrhythmia.

Conundrum of vitamin D on glucose and fuel homeostasis

As an endocrine hormone, vitamin D plays an important role in bone health and calcium homeostasis. Over the past two decades, the non-calcemic effects of vitamin D were extensively examined. Although the effect of vitamin D on beta cell function were known for some time, the effect of vitamin D on glucose and fuel homeostasis has attracted new interest among researchers. Yet, to date, studies remain inconclusive and controversial, in part, due to a lack of understanding of the threshold effects of vitamin D. In this review, a critical examination of interventional trials of vitamin D in prevention of diabetes is provided. Like use of vitamin D for bone loss, the benefits of vitamin D supplementation in diabetes prevention were observed in vitamin D-deficient subjects with serum 25-hydroxyvitamin D < 50 nmol/L (20 ng/mL). The beneficial effect from vitamin D supplementation was not apparent in subjects with serum 25-hydroxyvitamin D > 75 nmol/L (30 ng/mL). Furthermore, no benefit was noted in subjects that achieved serum 25-hydroxyvitamin D > 100 nmol/L (40 ng/mL). Further studies are required to confirm these observations.

Decreased Expression of Ileal Thyroid Hormone Transporters in a Hypothyroid Patient: A Case Report

BACKGROUND

Malabsorption of levothyroxine (LT4) is a common problem faced in clinical practice. It is usually solved, if there are no complexities including gastrointestinal absorption disorder, by taking medicines on an empty stomach and avoiding foods interfering with LT4. Herein we present a rare case of a patient exhibiting malabsorption of LT4 with decreased membranous expression of ileal transporters.

CASE

The 22-Year-old female presented with sustained hypothyroid status despite medication of 7.8 μg/kg LT4. Medical history and LT4 absorption test (the absorption rate 8.4%) excluded pseudomalabsorption. No organic gastrointestinal disorder was found in the patient by blood chemistry, endoscopies, and abdominal computed tomography scan. The immunohistochemical analysis showed decreased membranous expression of LAT1 and LAT2 in distal ileum and ascending colon in the patient compared to 20 controls who have no thyroid disease. The expression of MCT8 in colon appeared at both nucleus and brush border in the patient, while it was limited to brush border in controls. The expression of other transporters was similar between the patient and controls.

CONCLUSION

The changes of the expression of LAT1 and LAT2 in this patient showing LT4 malabsorption might help to understand the role of intestinal transporters in the absorption of LT4 in humans. The functional relevance of the decrement of LAT1 and LAT2 in this patient remains to be elucidated.

Higher Caloric Exposure in Critically Ill Patients Transiently Accelerates Thyroid Hormone Activation

INTRODUCTION

The inflammatory response of critical illness is accompanied by nonthyroidal illness syndrome (NTIS). Feeding has been shown to attenuate this process, but this has not been explored prospectively over time in critically ill patients.

OBJECTIVE

To explore the impact of calorie exposure on NTIS over time in critically ill patients.

METHODS

Mechanically ventilated patients with systemic inflammatory response syndrome (SIRS) were randomized to receive either 100% or 40% of their estimated caloric needs (ECN). Thyroid hormones were measured daily for 7 days or until intensive care unit discharge or death. Mixed level regression modeling was used to explore the effect of randomization group on plasma triiodothyronine (T3), reverse triiodothyronine (rT3), thyroxine (T4), and thyroid stimulating hormone (TSH), as well as the T3/rT3 ratio.

RESULTS

Thirty-five participants (n=19 in 100% ECN; n=16 in 40% ECN) were recruited. Adjusting for group differences in baseline T3/rT3 ratio, the parameters defining the fitted curves (intercept, linear effect of study day, and quadratic effect of study day) differed by randomization group (P = 0.001, P = 0.01, and P = 0.02 respectively). Plots of the fitted curves revealed that participants in the 100% ECN group had a 54% higher T3/rT3 ratio on postintervention day 1 compared with the 40% ECN group, a difference which attenuated over time. This was driven by a 23% higher plasma T3 and 10% lower plasma rT3 levels on postintervention 1.

CONCLUSIONS

Higher caloric exposure in NTIS patients transiently attenuates the drop of the plasma T3/rT3 ratio, an effect that is minimized and finally lost over the following 3 days of continued higher caloric exposure.

Pharmacokinetics of L-Triiodothyronine in Patients Undergoing Thyroid Hormone Therapy Withdrawal

Background: L-triiodothyronine (LT3) is a substitute for levothyroxine (LT4) for thyroid cancer (TC) patients during the preparation for nuclear medicine procedures, and it is used in combination with LT4 in patients who do not respond to the standard treatment for hypothyroidism. This therapy is commonly done by using fixed doses, potentially resulting in supraphysiologic levels of triiodothyronine (T3). A good understanding of the LT3 pharmacokinetics (PK) is necessary to design combination treatment schemes that are able to maintain serum T3 levels within the reference range, but data on the PK of LT3 are conflicting. Here, we present a study designed to characterize the PK of LT3 in patients devoid of endogenous thyroid hormone production, and not receiving LT4 therapy. Methods: We performed an open-label, PK study in patients undergoing thyroid hormone withdrawal in preparation for nuclear medicine procedures for the evaluation and treatment of follicular-derived TC. LT3 was substituted for LT4 at a 1:3 mcg/mcg dosage ratio thrice daily for at least 30 days. PK of the last LT3 dose while at steady state and terminal elimination was assessed over 11 days. Thereafter, a PK study was performed following the nuclear medicine procedure in patients who volunteered for a second study. Results: Fourteen patients age 48.5 ± 16.0 years completed the last dose study and five completed the second PK study. PK analysis indicates a time to maximum serum concentration of 1.8 ± 0.32 hours and two distinct phases of linear elimination, with a fast distribution phase and slow elimination phases with half-lives of 2.3 ± 0.11 hours and 22.9 ± 7.7 hours, supporting a two-compartment model. PK modeling predicts that a twice-daily administration of low-dose LT3 (0.07 mcg/kg twice daily) in combination with LT4 can predictably increase the serum T3 concentration without significant peaks above the reference range. Conclusions: The PK of LT3 is well described by a two-compartment model that assumes elimination only from the sampling compartment, with a rapid distribution phase and a slow elimination phase. This information will contribute to design therapeutic strategies for LT3/LT4 combination therapies directed to maintain stable T3 serum levels.

The role of thyroglobulin in thyroid hormonogenesis

In humans, the thyroid hormones T₃ and T₄ are synthesized in the thyroid gland in a process that crucially involves the iodoglycoprotein thyroglobulin. The overall structure of thyroglobulin is conserved in all vertebrates. Upon thyroglobulin delivery from thyrocytes to the follicular lumen of the thyroid gland via the secretory pathway, multiple tyrosine residues can become iodinated to form mono-iodotyrosine (MIT) and/or di-iodotyrosine (DIT); however, selective tyrosine residues lead to preferential formation of T₄ and T₃ at distinct sites. T₄ formation involves oxidative coupling between two DIT side chains, and de novo T₃ formation involves coupling between an MIT donor and a DIT acceptor. Thyroid hormone synthesis is stimulated by TSH activating its receptor (TSHR), which upregulates the activity of many thyroid gene products involved in hormonogenesis. Additionally, TSH regulates post-translational changes in thyroglobulin that selectively enhance its capacity for T₃ formation - this process is important in iodide deficiency and in Graves disease. 167 different mutations, many of which are newly discovered, are now known to exist in TG (encoding human thyroglobulin) that can lead to defective thyroid hormone synthesis, resulting in congenital hypothyroidism.

The clinical utility of free thyroxine in oral levothyroxine absorption testing

OBJECTIVE

Original absorption studies for levothyroxine (LT4) were validated using total thyroxine (TT4) measurements. Free thyroxine (FT4) has largely supplanted TT4 in clinical practice. The objective of our study was to assess the clinical utility of FT4 in oral LT4 absorption testing.

METHODS

In this retrospective electronic health record analysis, we recorded data of patients who underwent LT4 oral absorption testing between November 2010 and January 2012 because of persistent hypothyroidism despite a greater than anticipated weight-based dose of LT4. Patients included had primary hypothyroidism and an absorption test with assessment of both TT4 and FT4 measured at times 0, 30, 60, 90, 120, 180, 240, 300, and 360 minutes. The test was conducted with 1 mg (five 200-μg tablets) of Synthroid® after an overnight fast by a standard nonisotopic method.

RESULTS

A total of 10 patients (3 men/7 women) underwent absorption testing. Prior to testing, the median daily LT4 dose was 250 μg (range, 150 to 350 μg). Three patients were also on liothyronine (10, 20, or 50 μg daily). Based on the calculated amount absorbed, 1 patient demonstrated subnormal absorption, and 9 patients were normal. Median body mass index was 33 kg/m2 (range, 21 to 50 kg/m2). Median calculated absorption was 105% (range, 3.7 to 195.6%). The correlation comparing FT4 and TT4 was 0.88 (95% confidence interval, 0.56 to 0.97; P<.001), a significant correlation.

CONCLUSION

FT4 and TT4 correlated highly, even in patients who were severely hypothyroid; FT4 may be used interchangeably with TT4 as a qualitative assessment of suspected malabsorption using an oral LT4 absorption test.

Gastrointestinal Malabsorption of Thyroxine

Levothyroxine, a largely prescribed drug with a narrow therapeutic index, is often a lifelong treatment. The therapeutic efficacy of T4 may be marred by behavioral, pharmacologic, and pathologic issues acting as interfering factors. Despite a continuous search for an optimal T4 treatment, a significant number of patients fail to show a complete chemical and/or clinical response to this reference dose of T4. Gastrointestinal malabsorption of oral T4 represents an emerging cause of refractory hypothyroidism and may be more frequent than previously reputed. In this review, we examine the pharmacologic features of T4 preparations and their linkage with the intestinal absorption of the hormone. We have stressed the major biochemical and pharmacologic characteristics of T4 and its interaction with the putative transporter at the intestinal level. We have examined the interfering role of nutrients, foods, and drugs on T4 absorption at the gastric and intestinal levels. The impact of gastrointestinal disorders on T4 treatment efficacy has been also analyzed, in keeping with the site of action and the interfering mechanisms. Based on the evidence obtained from the literature, we also propose a schematic diagnostic workup for the most frequent and often hidden gastrointestinal diseases impairing T4 absorption.

Pharmacokinetics and Comparative Bioavailability of a Levothyroxine Sodium Oral Solution and Soft Capsule

A new formulation of levothyroxine sodium has been developed in the form of an oral solution contained in unit‐dose ampules. A study has been conducted to compare the bioavailability of levothyroxine sodium oral solution and levothyroxine sodium soft capsule in healthy volunteers under fasting conditions. The rate and extent of absorption of the new levothyroxine solution were also evaluated when administered on dilution in water or directly into the mouth without water. In each period, according to the randomization scheme, subjects were administered single oral doses of either test, as 4 × 150‐μg unit‐dose ampules, with or without water, or reference, as 4 × 150‐μg capsules in a crossover design. Thirty‐six subjects were randomized and dosed in this study; of these, 31 completed all study periods. When comparing the solution with the capsule (both products administered with water), the 90% confidence intervals for the ratio of log‐transformed values of AUC_0‐48 and C_max were within 90.00% and 111.11%, respectively, for baseline‐corrected levothyroxine. Moreover, the administration of levothyroxine oral solution without water did not affect the rate and extent of its absorption. In conclusion, levothyroxine oral solution unit‐dose ampules were bioequivalent to the levothyroxine capsule when administered with or without water. All formulations were well tolerated, with no major side effects.

Current evidence for the treatment of hypothyroidism with levothyroxine/levotriiodothyronine combination therapy versus levothyroxine monotherapy

OBJECTIVE

Hypothyroidism is relatively common, occurring in approximately 5% of the general US population aged ≥12 years. Levothyroxine (LT4) monotherapy is the standard of care. Approximately, 5%-10% of patients who normalise thyroid-stimulating hormone levels with LT4 monotherapy may have persistent symptoms that patients and clinicians may attribute to hypothyroidism. A long-standing debate in the literature is whether addition of levotriiodothyronine (LT3) to LT4 will ameliorate lingering symptoms. Here, we explore the evidence for and against LT4/LT3 combination therapy as the optimal approach to treat euthyroid patients with persistent complaints.

METHODS

Recent literature indexed on PubMed was searched in March 2017 using the terms "hypothyroid" or "hypothyroidism" and "triiodothyronine combination" or "T3 combination." Relevant non-review articles published in English during the past 10 years were included and supplemented with articles already known to the authors.

FINDINGS

Current clinical evidence is not sufficiently strong to support LT4/LT3 combination therapy in patients with hypothyroidism. Polymorphisms in deiodinase genes that encode the enzymes that convert T4 to T3 in the periphery may provide potential mechanisms underlying unsatisfactory treatment results with LT4 monotherapy. However, results of studies on the effect of LT4/LT3 therapy on clinical symptoms and thyroid-responsive genes have thus far not been conclusive.

CONCLUSIONS

Persistent symptoms in patients who are biochemically euthyroid with LT4 monotherapy may be caused by several other conditions unrelated to thyroid function, and their cause should be aggressively investigated by the clinician.

Stable Isotope Pharmacokinetic Studies Provide Insight into Effects of Age, Sex, and Weight on Levothyroxine Metabolism

BACKGROUND

This study sought to determine whether levothyroxine pharmacokinetics (PKs) are affected by age, weight, and sex.

METHODS

A PK study was performed after administration of a tracer dose of carbon-13-labeled LT4 (¹³C-LT4). The study was conducted at an academic medical center. Adults of any age being treated with levothyroxine for hypothyroidism were enrolled in the study. A single dose of ¹³C-LT4 was administered. Eighteen serial plasma samples were collected. One sample was obtained before the ¹³C-LT4 dose, and the majority of the remaining samples were collected over the 120-hour period post dosing. ¹³C-LT4 concentration was quantified using liquid chromatography tandem mass spectrometry. PK analysis was conducted using a linear log trapezoidal non-compartmental analysis using Phoenix 6.4.

RESULTS

Eight males and 33 females with a median age of 50 years (range 22-78 years) and median weight of 65.9 kg (range 50-150 kg) were enrolled in the study. The median ¹³C-LT4 dose administered was 100 μg (range 70-300 μg). The median oral clearance rate (CL/F), apparent volume of distribution (V/F), time to peak concentration (T_max), and dose-normalized peak concentration (C_max) of ¹³C-LT4 were estimated to be 0.712 L/h, 164.9 L, 4 h, and 7.5 ng/L/μg, respectively. The dose-normalized area under the concentration-time curve from time 0 to 120 hours and half-life of the terminal distribution phase were 0.931 ng.h/mL/μg and 172.2 h, respectively. There was no significant difference in any ¹³C-LT4 PK parameter between patients aged >60 years (n = 10) and patients aged ≤60 years (n = 31), nor was there a relationship between age as a continuous variable and ¹³C-LT4 PK parameters. Sex only affected CL/F, V/F, and dose-normalized C_max in univariate analyses. However, after adjusting for weight, sex was no longer a significant covariate. Weight was a significant predictor for CL/F, V/F and dose-normalized C_max of ¹³C-LT4 in multivariate analyses.

CONCLUSION

Prior studies suggest that patient age affects levothyroxine dose requirement. This study did not identify an effect of age and suggests that age-related changes in levothyroxine pharmacokinetics may be mediated by age-related weight differences. Physicians should consider a patient's weight, rather than age, for estimating levothyroxine dosage requirement.

Estimation of iodine nutrition and thyroid function status in late-gestation pregnant women in the United States: Development and application of a population-based pregnancy model

Previously, a deterministic biologically-based dose-response (BBDR) pregnancy model was developed to evaluate moderate thyroid axis disturbances with and without thyroid-active chemical exposure in a near-term pregnant woman and fetus. In the current study, the existing BBDR model was adapted to include a wider functional range of iodine nutrition, including more severe iodine deficiency conditions, and to incorporate empirically the effects of homeostatic mechanisms. The extended model was further developed into a population-based model and was constructed using a Monte Carlo-based probabilistic framework. In order to characterize total (T4) and free (fT4) thyroxine levels for a given iodine status at the population-level, the distribution of iodine intake for late-gestation pregnant women in the U.S was reconstructed using various reverse dosimetry methods and available biomonitoring data. The range of median (mean) iodine intake values resulting from three different methods of reverse dosimetry tested was 196.5-219.9μg of iodine/day (228.2-392.9μg of iodine/day). There was minimal variation in model-predicted maternal serum T4 and ft4 thyroxine levels from use of the three reconstructed distributions of iodine intake; the range of geometric mean for T4 and fT4, was 138-151.7nmol/L and 7.9-8.7pmol/L, respectively. The average value of the ratio of the 97.5th percentile to the 2.5th percentile equaled 3.1 and agreed well with similar estimates from recent observations in third-trimester pregnant women in the U.S. In addition, the reconstructed distributions of iodine intake allowed us to estimate nutrient inadequacy for late-gestation pregnant women in the U.S. via the probability approach. The prevalence of iodine inadequacy for third-trimester pregnant women in the U.S. was estimated to be between 21% and 44%. Taken together, the current work provides an improved tool for evaluating iodine nutritional status and the corresponding thyroid function status in pregnant women in the U.S. This model enables future assessments of the relevant risk of thyroid hormone level perturbations due to exposure to thyroid-active chemicals at the population-level.

Thermoregulatory and Cardiovascular Consequences of a Transient Thyrotoxicosis and Recovery in Male Mice

Thyroid hormones play a major role in body homeostasis, regulating energy expenditure and cardiovascular function. Given that obese people or athletes might consider rapid weight loss as beneficial, voluntary intoxication with T4 preparations is a growing cause for thyrotoxicosis. However, the long-lasting effects of transient thyrotoxicosis are poorly understood. Here we examined metabolic, thermoregulatory, and cardiovascular function upon induction and recovery from a 2-week thyrotoxicosis in male C57BL/6J mice. Our results showed that T4 treatment caused tachycardia, decreased hepatic glycogen stores, and higher body temperature as expected; however, we did not observe an increase in brown fat thermogenesis or decreased tail heat loss, suggesting that these tissues do not contribute to the hyperthermia induced by thyroid hormone. Most interestingly, when the T4 treatment was ended, a pronounced bradycardia was observed in the animals, which was likely caused by a rapid decline of T3 even below baseline levels. On the molecular level, this was accompanied by an overexpression of cardiac phospholamban and Serca2a mRNA, supporting the hypothesis that the heart depends more on T3 than T4. Our findings therefore demonstrate that a transient thyrotoxicosis can have pathological effects that even persist beyond the recovery of serum T4 levels, and in particular the observed bradycardia could be of clinical relevance when treating hyperthyroid patients.

Estimating Margin of Exposure to Thyroid Peroxidase Inhibitors Using High-Throughput in vitro Data, High-Throughput Exposure Modeling, and Physiologically Based Pharmacokinetic/Pharmacodynamic Modeling

Some pharmaceuticals and environmental chemicals bind the thyroid peroxidase (TPO) enzyme and disrupt thyroid hormone production. The potential for TPO inhibition is a function of both the binding affinity and concentration of the chemical within the thyroid gland. The former can be determined through in vitro assays, and the latter is influenced by pharmacokinetic properties, along with environmental exposure levels. In this study, a physiologically based pharmacokinetic (PBPK) model was integrated with a pharmacodynamic (PD) model to establish internal doses capable of inhibiting TPO in relation to external exposure levels predicted through exposure modeling. The PBPK/PD model was evaluated using published serum or thyroid gland chemical concentrations or circulating thyroxine (T4) and triiodothyronine (T3) hormone levels measured in rats and humans. After evaluation, the model was used to estimate human equivalent intake doses resulting in reduction of T4 and T3 levels by 10% (ED10) for 6 chemicals of varying TPO-inhibiting potencies. These chemicals were methimazole, 6-propylthiouracil, resorcinol, benzophenone-2, 2-mercaptobenzothiazole, and triclosan. Margin of exposure values were estimated for these chemicals using the ED10 and predicted population exposure levels for females of child-bearing age. The modeling approach presented here revealed that examining hazard or exposure alone when prioritizing chemicals for risk assessment may be insufficient, and that consideration of pharmacokinetic properties is warranted. This approach also provides a mechanism for integrating in vitro data, pharmacokinetic properties, and exposure levels predicted through high-throughput means when interpreting adverse outcome pathways based on biological responses.

Single-dose T3 administration: kinetics and effects on biochemical and physiological parameters

BACKGROUND

As changes in thyroid stimulating hormone (TSH), thyroid hormones, and vital signs after administration of a single dose of liothyronine have typically only been documented for 24 hours, we documented these parameters more than 96 hours.

METHODS

Blood samples were obtained for 4 days after administration of 50-mcg liothyronine to 12 healthy euthyroid participants. Concentrations of total and free triiodothyronine, free and total thyroxine, and TSH were measured. Vital signs were documented.

RESULTS

Triiodothyronine concentrations peaked at 2.5 hours after liothyronine administration. Heart rate (HR) increased by 5 hours after liothyronine administration, subsequently reaching a value higher than baseline (P = 0.009). Suppression of TSH concentrations began at 2 hours. The nadir TSH value at 12 hours was significantly different from baseline (P < 0.001) and remained lower than the baseline value for 2-3 days.

CONCLUSIONS

A single dose of liothyronine has both short-term and long-term effects. There is clearly a different lag time between the serum concentrations of triiodothyronine and its effects on the heart and pituitary, respectively. The increase in serum triiodothyronine concentration occurred within hours and was then followed by an increase in HR. The increased HR was transient and was followed by a reduction in TSH concentration. The suppression of TSH was delayed but was more sustained. Thus, sustained TSH reduction beyond 24 hours was achieved by a single dose of liothyronine that produced only brief increases in serum triiodothyronine levels and transient increases in HR.

Quantitative global sensitivity analysis of a biologically based dose-response pregnancy model for the thyroid endocrine system

A deterministic biologically based dose-response model for the thyroidal system in a near-term pregnant woman and the fetus was recently developed to evaluate quantitatively thyroid hormone perturbations. The current work focuses on conducting a quantitative global sensitivity analysis on this complex model to identify and characterize the sources and contributions of uncertainties in the predicted model output. The workflow and methodologies suitable for computationally expensive models, such as the Morris screening method and Gaussian Emulation processes, were used for the implementation of the global sensitivity analysis. Sensitivity indices, such as main, total and interaction effects, were computed for a screened set of the total thyroidal system descriptive model input parameters. Furthermore, a narrower sub-set of the most influential parameters affecting the model output of maternal thyroid hormone levels were identified in addition to the characterization of their overall and pair-wise parameter interaction quotients. The characteristic trends of influence in model output for each of these individual model input parameters over their plausible ranges were elucidated using Gaussian Emulation processes. Through global sensitivity analysis we have gained a better understanding of the model behavior and performance beyond the domains of observation by the simultaneous variation in model inputs over their range of plausible uncertainties. The sensitivity analysis helped identify parameters that determine the driving mechanisms of the maternal and fetal iodide kinetics, thyroid function and their interactions, and contributed to an improved understanding of the system modeled. We have thus demonstrated the use and application of global sensitivity analysis for a biologically based dose-response model for sensitive life-stages such as pregnancy that provides richer information on the model and the thyroidal system modeled compared to local sensitivity analysis.

Guidelines for the treatment of hypothyroidism: prepared by the american thyroid association task force on thyroid hormone replacement

BACKGROUND

A number of recent advances in our understanding of thyroid physiology may shed light on why some patients feel unwell while taking levothyroxine monotherapy. The purpose of this task force was to review the goals of levothyroxine therapy, the optimal prescription of conventional levothyroxine therapy, the sources of dissatisfaction with levothyroxine therapy, the evidence on treatment alternatives, and the relevant knowledge gaps. We wished to determine whether there are sufficient new data generated by well-designed studies to provide reason to pursue such therapies and change the current standard of care. This document is intended to inform clinical decision-making on thyroid hormone replacement therapy; it is not a replacement for individualized clinical judgment.

METHODS

Task force members identified 24 questions relevant to the treatment of hypothyroidism. The clinical literature relating to each question was then reviewed. Clinical reviews were supplemented, when relevant, with related mechanistic and bench research literature reviews, performed by our team of translational scientists. Ethics reviews were provided, when relevant, by a bioethicist. The responses to questions were formatted, when possible, in the form of a formal clinical recommendation statement. When responses were not suitable for a formal clinical recommendation, a summary response statement without a formal clinical recommendation was developed. For clinical recommendations, the supporting evidence was appraised, and the strength of each clinical recommendation was assessed, using the American College of Physicians system. The final document was organized so that each topic is introduced with a question, followed by a formal clinical recommendation. Stakeholder input was received at a national meeting, with some subsequent refinement of the clinical questions addressed in the document. Consensus was achieved for all recommendations by the task force.

RESULTS

We reviewed the following therapeutic categories: (i) levothyroxine therapy, (ii) non-levothyroxine-based thyroid hormone therapies, and (iii) use of thyroid hormone analogs. The second category included thyroid extracts, synthetic combination therapy, triiodothyronine therapy, and compounded thyroid hormones.

CONCLUSIONS

We concluded that levothyroxine should remain the standard of care for treating hypothyroidism. We found no consistently strong evidence for the superiority of alternative preparations (e.g., levothyroxine-liothyronine combination therapy, or thyroid extract therapy, or others) over monotherapy with levothyroxine, in improving health outcomes. Some examples of future research needs include the development of superior biomarkers of euthyroidism to supplement thyrotropin measurements, mechanistic research on serum triiodothyronine levels (including effects of age and disease status, relationship with tissue concentrations, as well as potential therapeutic targeting), and long-term outcome clinical trials testing combination therapy or thyroid extracts (including subgroup effects). Additional research is also needed to develop thyroid hormone analogs with a favorable benefit to risk profile.

A Case of simultaneous occurrence of Marine - Lenhart syndrome and a papillary thyroid microcarcinoma

BACKGROUND

Marine-Lenhart syndrome is defined as the co-occurrence of Graves' disease and functional nodules. The vast majority of autonomous adenomas are benign, whereas functional thyroid carcinomas are considered to be rare. Here, we describe a case of simultaneous occurrence of Marine-Lenhart syndrome and a papillary microcarcinoma embedded in a functional nodule.

CASE PRESENTATION

A 55 year-old, caucasian man presented with overt hyperthyroidism (thyrotropin (TSH) <0.01 μIU/L; free thyroxine (FT4) 3.03 ng/dL), negative thyroid peroxidase and thyroglobulin autoantibodies, but elevated thyroid stimulating hormone receptor antibodies (TSH-RAb 2.6 IU/L). Ultrasound showed a highly vascularized hypoechoic nodule (1.1 × 0.9 × 2 cm) in the right lobe, which projected onto a hot area detected in the 99mtechnetium thyroid nuclear scan. Overall uptake was increased (4.29%), while the left lobe showed lower tracer uptake with no visible background-activity, supporting the notion that both Graves' disease and a toxic adenoma were present. After normal thyroid function was reinstalled with methimazole, the patient underwent thyroidectomy. Histological work up revealed a unifocal papillary microcarcinoma (9 mm, pT1a, R0), positively tested for the BRAF V600E mutation, embedded into the hyperfunctional nodular goiter.

CONCLUSIONS

Neither the finding of an autonomously functioning thyroid nodule nor the presence of Graves' disease rule out papillary thyroid carcinoma.

Systems pharmacology modeling of drug-induced modulation of thyroid hormones in dogs and translation to human

PURPOSE

To develop a systems pharmacology model based on hormone physiology and pharmacokinetic-pharmacodynamic concepts describing the impact of thyroperoxidase (TPO) inhibition on thyroid hormone homeostasis in the dog and to predict drug-induced changes in thyroid hormones in humans.

METHODS

A population model was developed based on a simultaneous analysis of concentration-time data of T₄, T₃ and TSH in dogs following once daily oral dosing for up to 6-months of a myeloperoxidase inhibitor (MPO-IN1) with TPO inhibiting properties. The model consisted of linked turnover compartments for T₄, T₃ and TSH including a negative feedback from T₄ on TSH concentrations.

RESULTS

The model could well describe the concentration-time profiles of thyroid hormones in dog. Successful model validation was performed by predicting the hormone concentrations during 1-month administration of MPO-IN2 based on its in vitro dog TPO inhibition potency. Using human thyroid hormone turnover rates and TPO inhibitory potency, the human T₄ and TSH concentrations upon MPO-IN1 treatment were predicted well.

CONCLUSIONS

The model provides a scientific framework for the prediction of drug induced effects on plasma thyroid hormones concentrations in humans via TPO inhibition based on results obtained in in vitro and animal studies.

A Review of the Pharmacokinetics of Levothyroxine for the Treatment of Hypothyroidism

Thyroxine hormone has been recognised since the early part of the nineteenth century and levothyroxine has been available since the mid-nineteenth century as a replacement for deficient thyroid hormones. While levothyroxine remains the staple treatment for hypothyroidism even to this day, its optimal use can be challenging. As is often the case with older drugs, the pharmacokinetics of levothyroxine is often under-appreciated or misunderstood and many factors influence the optimal dosing of levothyroxine. This article will review the pharmacokinetics of levothyroxine in the treatment of hypothyroidism and highlight major concepts that should aid both clinicians and researchers.

TSH and Thyrotropic Agonists: Key Actors in Thyroid Homeostasis

This paper provides the reader with an overview of our current knowledge of hypothalamic-pituitary-thyroid feedback from a cybernetic standpoint. Over the past decades we have gained a plethora of information from biochemical, clinical, and epidemiological investigation, especially on the role of TSH and other thyrotropic agonists as critical components of this complex relationship. Integrating these data into a systems perspective delivers new insights into static and dynamic behaviour of thyroid homeostasis. Explicit usage of this information with mathematical methods promises to deliver a better understanding of thyrotropic feedback control and new options for personalised diagnosis of thyroid dysfunction and targeted therapy, also by permitting a new perspective on the conundrum of the TSH reference range.

THE SYNTHESIS OF 13C9-15N-LABELED 3,5-DIIODOTHYRONINE AND THYROXINE

Thyroid hormones undergo extensive metabolism to regulate hormone activity. A labeled thyroid hormone would be useful to track hormone metabolism through various pathways. While radiolabeled thyroid hormones have been synthesized and used for in vivo studies, a stable isotope labeled form of thyroid hormone is required for studying thyroid hormone metabolism by LC-MS/MS, an analytical technique that has certain advantages without the complications of radioactivity. Here we report the synthesis of 13C9-15N-T2 and 13C9-15N-T4, two labeled thyroid hormone derivatives suitable for in vivo LC-MS/MS studies.

Extrapolation of hypothalamic-pituitary-thyroid axis perturbations and associated toxicity in rodents to humans: case study with perchlorate

Functional aspects of the Hypothalamic-Pituitary-Thyroid (HPT) axis in rats and humans are compared, exposing why extrapolation of toxicant-induced perturbations in the rat HPT axis to the human HPT axis cannot be accomplished using default risk assessment methodology. Computational tools, such as biologically based dose response models for the HPT axis, are recommended to perform complex animal to human extrapolations involving the HPT axis. Experimental and computational evidence are presented that suggest perchlorate acts directly on the thyroid gland in rats. The apparent escape from perchlorate-induced inhibition of thyroidal uptake of radioactive iodide in humans is discussed along with "rebound" or increased thyroidal uptake of radioactive iodide observed after discontinued clinical treatment with perchlorate.

Xanthohumol, a prenylated chalcone from hops, modulates hepatic expression of genes involved in thyroid hormone distribution and metabolism

In the present study, we analyzed the influence of xanthohumol (XN) on thyroid hormone (TH) distribution and metabolism in rats. A potent and selective competition of XN for thyroxine (T4) binding to transthyretin (IC(50)=1 microM at 1.7 nM [(125)I]T4) was found in human and rat sera in vitro. Female rats treated orally with XN showed increased hepatic expression of T4-binding globulin and decreased transthyretin and albumin. Thyrotropin levels and hepatic type 1 deiodinase activity were moderately increased. Northern blot analysis revealed diminished expression of liver sulfotransferase (Sult1a1) and uridine-diphosphate glucuronosyltransferase (Ugt1a1) after XN treatment. The transcript levels of constitutive androstane receptor (CAR), known to be involved in regulation of enzymes metabolizing hormones, drugs and xenobiotics, was lower in rats treated with >10 mg XN/kg body weight per day. Immunoblot analysis indicates reduced amounts of CAR protein. The phenobarbital-inducible cytochrome P450 mRNA level was decreased in rats treated with >10 mg XN/kg/day, in agreement with reduced CAR protein. Although only moderate changes in TH serum levels were observed, the XN-dependent altered expression of components involved in TH homeostasis might be important not only for hormone metabolism, but also for hepatic phase I and II elimination of drug metabolites and xenobiotics.

Conditions and drugs interfering with thyroxine absorption

Food, dietary fibre and espresso coffee interfere with the absorption of levothyroxine. Malabsorptive disorders reported to affect the absorption of levothyroxine include coeliac disease, inflammatory bowel disease, lactose intolerance as well as Helicobacter pylori (H. pylori) infection and atrophic gastritis. Many commonly used drugs, such as bile acid sequestrants, ferrous sulphate, sucralfate, calcium carbonate, aluminium-containing antacids, phosphate binders, raloxifene and proton-pump inhibitors, have also been shown to interfere with the absorption of levothyroxine.

The effects of long-term antithyroid drug treatment on serum reverse T3 in patients with Graves' disease

The effects of long-term treatment with antithyroid drugs, carbimazole (CMI) or propylthiouracil (PTU), on serum reverse triiodothyronine (rT3) levels were studied in 23 patients with Graves' disease. Nineteen patients were given CMI and four PTU for a minimum of six months. After one month of treatment the serum levels of thyroxine (T4), triiodothyronine (T3) and rT3 had normalized in both groups. When L-thyroxine was added to the regimens after two months of therapy, both serum T4 and rT3 levels increased, whereas serum T3 level continued to fall. The serum levels of rT3 seemed to be dependent on and followed the T4 levels so closely that determinations of rT3 in the medical management of patients with Graves' disease will be of little clinical use.

The influence of propranolol on the extrathyroidal metabolism of 3,3',5'-triiodothyronine (reverse T3)

The effect of propranolol 80 mg daily on the metabolism of 3,3',5'-triiodothyronine (reverse T3, rT3), 3,3',5'-triiodothyronine (T3) and thyroxine (T4) was studied by means of a non compartmental kinetic method in seven females with severe pretreatment hypothyroidism. The patients were maintained euthyroid on a constant L-T4 replacement therapy. Serum rT3 levels increased significantly during propranolol (p less than 0.02). This increase was explained by a decrease in metabolic clearance rate (MCR) (p less than 0.02), since the conversion rate from T4 and the distribution volume of rT3 were unchanged. By contrast the decreased serum levels of T3 were due to a significant decreased conversion from T4 (p less than 0.02) in spite of a decreased MCR. The results are compatible with the assumption of two different monodeiodinating enzymes, a 5-deiodinase responsible for the diodination of T4 to rT3 and a 5'-deiodinase responsible for the deiodination of T4 to T3.

Multigenic control of thyroid hormone functions in the nervous system

Thyroid hormone (TH) has a remarkable range of actions in the development and function of the nervous system. A multigenic picture is emerging of the mechanisms that specify these diverse functions in target tissues. Distinct responses are mediated by alpha and beta isoforms of TH receptor which act as ligand-regulated transcription factors. Receptor activity can be regulated at several levels including that of uptake of TH ligand and the activation or inactivation of ligand by deiodinase enzymes in target tissues. Processes under the control of TH range from learning and anxiety-like behaviour to sensory function. At the cellular level, TH controls events as diverse as axonal outgrowth, hippocampal synaptic activity and the patterning of opsin photopigments necessary for colour vision. Overall, TH coordinates this variety of events in both central and sensory systems to promote the function of the nervous system as a complete entity.

Sources of circulating 3,5,3'-triiodothyronine in hyperthyroidism estimated after blocking of type 1 and type 2 iodothyronine deiodinases

CONTEXT

Graves' hyperthyroidism and multinodular toxic goiter lead to high serum T(3) compared with serum T(4). The source of this high T(3) has not been clarified.

OBJECTIVE

Our objective was to assess the role of iodothyronine deiodinase type 1 (D1) and type 2 (D2) for T(3) production and to estimate the sources of T(3) in hyperthyroidism.

DESIGN AND SETTING

The study was a prospective, randomized, open-labeled study in a secondary care setting.

PATIENTS AND METHODS

Consecutive patients with hyperthyroidism caused by Graves' disease or by multinodular toxic goiter were randomized to be treated with high-dose propylthiouracil (PTU) to block D1, PTU plus KI, or PTU plus sodium ipodate to additionally block D2. T(3) and T(4) were measured in serum, and we estimated the sources of T(3).

RESULTS

PTU reduced the T(3)/T(4) in serum to 47.7 +/- 2.5% (mean +/- sem) of the initial value on d 4 of therapy in patients with Graves' disease. The addition of KI to PTU led to a greater fall in T(3) and T(4), but the balance was unaltered. After PTU plus ipodate, T(3)/T(4) on d 4 was lower, 34.1 +/- 1.2% of the initial value. Similar variations were observed in patients with multinodular toxic goiter. Thus, the major source of the excess T(3) was D1-catalyzed T(4) deiodination, with a minor role for D2. It was estimated that the majority of this D1-catalyzed T(3) production takes place in the hyperactive thyroid gland.

CONCLUSION

Although thyroidal T(3) contributes only around 20% of total T(3) production in normal individuals, this is much higher in patients with a hyperactive thyroid, ranging up to two thirds. The major part is produced from T(4) deiodinated in the thyroid.

Triiodothyronine production in Graves' hyperthyroidism

OBJECTIVE

To quantify the relative contributions of thyroid secretion and peripheral generation to triiodothyronine (T(3)) production in untreated Graves' hyperthyroidism. PATIENTS, DESIGN, AND MEASUREMENTS: Thirty-one patients with hyperthyroidism, of whom 6 had T(3) toxicosis, and 21 surgically and radioiodine ablated patients with thyroid cancer on thyroid stimulating hormone-suppressive therapy in whom serum T(3) should reflect peripheral generation alone were compared with respect to serum free thyroxine (T(4)) and serum free T(3) concentrations.

MAIN OUTCOMES

Serum free T(4)/free T(3) molar ratios were virtually identical in the patients with T(4)+T(3) toxicosis (2.7 +/- 0.4) and those with T(3) toxicosis (2.6 +/- 0.4) and were significantly lower than in the patients with thyroid cancer (4.0 +/- 0.4) (p < 0.001). In the hyperthyroid patients, peripherally generated T(3) was calculated as the quotient of the individual serum free T(4) concentration and the free T(4)/free T(3) molar ratio in thyroid cancer; this value was subtracted from the individual measured free T(3) concentration to derive the value for secreted T3. Secreted T(3) accounted for 33 +/- 6% of T(3) production in T(4)+T(3) toxicosis and 34 +/- 10% in T(3) toxicosis.

CONCLUSIONS

This study indicates that about one third of T3 production in untreated Graves' hyperthyroidism, irrespective of whether presenting as T4+T3 toxicosis or T3 toxicosis, arises from thyroid secretion as compared to about 20% in normal individuals.

Substitution of liothyronine at a 1:5 ratio for a portion of levothyroxine: effect on fatigue, symptoms of depression, and working memory versus treatment with levothyroxine alone

OBJECTIVE

To attempt to confirm a previous report of superior effectiveness of using two thyroid hormones rather than one hormone to treat hypothyroidism.

METHODS

This trial attempted to replicate prior findings, which suggested that substituting 12.5 microg of liothyronine (LT(3)) for 50 microg of levothyroxine (LT(4)) might improve mood, cognition, and physical symptoms in patients with primary hypothyroidism. Additionally, this trial aimed to extend the previous findings to fatigue and to assess for differential effects in subjects with low fatigue and high fatigue at baseline. A randomized, double-blind, two-period, crossover design was used. At an endocrinology and diabetes clinic, 30 adult subjects with primary hypothyroidism stabilized on LT(4) were recruited. Patients randomly assigned to treatment sequence 1 received their standard LT(4) dose in one capsule and placebo in another. Patients assigned to sequence 2 received their usual LT(4) dose minus 50 microg in one capsule and 10 microg of LT(3) in the other. At the end of the first 6 weeks, subjects were crossed over to receive the other treatment. Carryover and treatment effects were assessed by t tests.

RESULTS

Of the 30 enrolled study subjects, 27 completed the trial. The mean LT(4) dose was 121 +/- 26 microg/day at baseline. No significant differences in fatigue and symptoms of depression were found between treatments. Measures of working memory were unchanged. During substitution treatment, the free thyroxine index was reduced by 0.7 (P<0.001), total serum thyroxine was reduced by 3.0 microg/dL (P<0.001), and total serum triiodothyronine was increased by 20.5 ng/dL (P = 0.004).

CONCLUSION

With regard to the outcomes measured, substitution of LT(3) at a 1:5 ratio for a portion of baseline LT(4) yielded no better results than did treatment with the original dose of LT(4) alone.

Thyrotoxicosis due to metastatic papillary thyroid cancer in a patient with Graves' disease

Thyrotoxicosis resulting from functional thyroid cancer metastases is extremely rare, and is mostly caused by follicular cancer. The lesions causing thyrotoxicosis are usually bulky and extensive. We report here a patient with Graves' disease and concomitant papillary thyroid cancer who developed metastases causing symptomatic thyrotoxicosis. His serum titers of thyroid stimulating Ig (TSIs) were elevated. We believe that TSIs were responsible for thyrotoxicosis by stimulating hormonogenesis in the metastatic lesions.