Effects of replacement doses of sodium L-thyroxine on the peripheral metabolism of thyroxine and triiodothyronine in man

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.

Role of thyroid hormone therapy in depressive disorders

PURPOSE

The close association among thyroid metabolism, mood disorders and behavior has long been known. The old and modern uses of thyroid hormones to modulate the expression of depression and bipolar disorder and to improve clinical outcome when used in conjunction with psychotropic medications.

METHODS

A literature search was performed to identify studies investigating the effects of thyroid hormone treatment in patient s with mood disorders.

RESULTS

The successful modification of mood disorders with thyroid hormone underscores the association between endocrine and cerebral systems in these disorders. Thyroid hormones have a profound influence on behavior and appear to be capable of modulating the phenotypic expression of major mood disorders. In fact, there is evidence that triiodothyronine (LT3) may accelerate the antidepressant response to antidepressants, and studies suggest that LT3 also may augment the response to antidepressants in refractory depression. Add-on treatment with supraphysiologic doses of levothyroxine (LT4) has shown efficacy in open-label and in placebo-controlled studies, including in rapid cycling and prophylaxis-resistant bipolar disorder, and with acute refractory uni- or bipolar depression. Functional brain-imaging studies (PET) demonstrated that administration of supraphysiologic LT4 improves depressive symptoms in patients with bipolar depression by modulating cerebral activity in the anterior limbic network.

CONCLUSION

The add-on administration of supraphysiologic doses of LT4 is a promising strategy in patients with refractory bipolar and depressive mood disorders.

Managing symptoms in hypothyroid patients on adequate levothyroxine: a narrative review

The current standard of care for hypothyroidism is levothyroxine (LT4) monotherapy to reduce levels of thyrotropin (thyroid-stimulating hormone, TSH) within its reference range and amelioration of any symptoms. A substantial minority continues to report hypothyroid-like symptoms despite optimized TSH, however. These symptoms are not specific to thyroid dysfunction and are frequent among the euthyroid population, creating a therapeutic dilemma for the treating clinician as well as the patient. We present a concise, narrative review of the clinical research and evidence-based guidance on the management of this challenging population. The clinician may endeavor to ensure that the serum TSH is within the target range. However, the symptomatic patient may turn to alternative non-evidence-based therapies in the hope of obtaining relief. Accordingly, it is important for the clinician to check for conditions unrelated to the thyroid that could account for the ongoing symptoms such as other autoimmune conditions, anemia or mental health disorders. Systematic and thorough investigation of the potential causes of persistent symptoms while receiving LT4 therapy will resolve the problem for most patients. There may be some patients that may benefit from additional treatment with liothyronine (LT3), although it is unclear as yet as to which patient group may benefit the most from combined LT4 + LT3 therapy. In the future, personalized treatment with LT4 + LT3 may be of benefit for some patients with persistent symptoms of hypothyroidism such as those with polymorphisms in the deiodinase enzyme 2 (DIO2). For now, this remains a subject for research.

Liothyronine and Desiccated Thyroid Extract in the Treatment of Hypothyroidism

Background: The basis for the treatment of hypothyroidism with levothyroxine (LT4) is that humans activate T4 to triiodothyronine (T3). Thus, while normalizing serum thyrotropin (TSH), LT4 doses should also restore the body's reservoir of T3. However, there is evidence that T3 is not fully restored in LT4-treated patients. Summary: For patients who remain symptomatic on LT4 therapy, clinical guidelines recommend, on a trial basis, therapy with LT4+LT3. Reducing the LT4 dose by 25 mcg/day and adding 2.5-7.5 mcg liothyronine (LT3) once or twice a day is an appropriate starting point. Transient episodes of hypertriiodothyroninemia with these doses of LT4 and LT3 are unlikely to go above the reference range and have not been associated with adverse drug reactions. Trials following almost a 1000 patients for almost 1 year indicate that similar to LT4, therapy with LT4+LT3 can restore euthyroidism while maintaining a normal serum TSH. An observational study of 400 patients with a mean follow-up of ∼9 years did not indicate increased mortality or morbidity risk due to cardiovascular disease, atrial fibrillation, or fractures after adjusting for age when compared with patients taking only LT4. Desiccated thyroid extract (DTE) is a form of combination therapy in which the LT4/LT3 ratio is ∼4:1; the mean daily dose of DTE needed to normalize serum TSH contains ∼11 mcg T3, but some patients may require higher doses. The DTE remains outside formal FDA oversight, and consistency of T4 and T3 contents is monitored by the manufacturers only. Conclusions: Newly diagnosed hypothyroid patients should be treated with LT4. A trial of combination therapy with LT4+LT3 can be considered for those patients who have unambiguously not benefited from LT4.

Differential Expression and Diagnostic Significance of Pre-Albumin, Fibrinogen Combined with D-Dimer in AFP-Negative Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the most malignant cancers with high morbidity and mortality. Nowadays, AFP-negative hepatocellular carcinoma (AFP-NHCC) has been found in many HCC patients and AFP analysis can't be used to screen HCC in these cases. In this study, we have examined the expression patterns of pre-albumin (PA), fibrinogen, D-Dimer and their clinical significance in AFP-NHCC. We recruited 214 AFP-NHCC patients and 210 controls in the study. PA, fibrinogen and D-Dimer levels were detected by turbidimetry, clauss and immunoturbidimetry methods, respectively. Serum PA levels were significantly lower in AFP-NHCC (84.5 ± 24.7 mg/L) than that in the controls (240.6 ± 59.4 mg/L, P < 0.05). For plasma fibrinogen levels, there was no difference between the controls (2.9 ± 0.7 g/L) and AFP-NHCC (2.5 ± 0.7 g/L). Compared with AFP-NHCC (0.8 ± 0.2 mg/L), plasma D-Dimer levels were significantly lower in controls (0.1 ± 0.0 mg/L, P < 0.05). The levels of PA, fibrinogen and D-Dimer were significantly correlated with differentiation (P < 0.01), and the PA and D-Dimer values were correlated with TNM stage (P < 0.05). Moreover, PA levels were correlated with tumor size (P = 0.034). Receiver operating characteristic curve (ROC) analyses elaborated that combination of PA, fibrinogen and D-Dimer possessed a higher sensitivity (93.4%) for differentiating AFP-NHCC from the controls, but the diagnostic specificity was reduced due to the combination of fibrinogen. After adjusting for all significant outcome predictors of the univariate logistic regression analysis, low levels of PA and high levels of D-Dimer were remained independent unfavorable outcome predictors (P < 0.05). Our data suggested that the expression levels of PA, fibrinogen and D-Dimer played critical roles in AFP-NHCC tumorigenesis. Moreover, PA and D-Dimer might be considered as potential diagnostic indicators in AFP-NHCC.

The Swinging Pendulum in Treatment for Hypothyroidism: From (and Toward?) Combination Therapy

Thyroid hormone replacement for hypothyroidism can be achieved via several approaches utilizing different preparations of thyroid hormones, T3, and/or T4. "Combination therapy" involves administration of both T3 and T4, and was technically the first treatment for hypothyroidism. It was lauded as a cure for the morbidity and mortality associated with myxedema, the most severe presentation of overt hypothyroidism. In the late nineteenth and the early Twentieth centuries, combination therapy per se could consist of thyroid gland transplant, or more commonly, consumption of desiccated animal thyroid, thyroid extract, or thyroglobulin. Combination therapy remained the mainstay of therapy for decades despite development of synthetic formulations of T4 and T3, because it was efficacious and cost effective. However, concerns emerged about the consistency and potency of desiccated thyroid hormone after cases were reported detailing either continued hypothyroidism or iatrogenic thyrotoxicosis. Development of the TSH radioimmunoassay and discovery of conversion of T4-to-T3 in humans led to a major transition in clinical practices away from combination therapy, to adoption of levothyroxine "monotherapy" as the standard of care. Levothyroxine monotherapy has a favorable safety profile and can effectively normalize the serum TSH, the most sensitive marker of hypothyroidism. Whether levothyroxine monotherapy restores thyroid hormone signaling within all tissues remains controversial. Evidence of persistent signs and symptoms of hypothyroidism during levothyroxine monotherapy at doses that normalize serum TSH is mounting. Hence, in the last decade there has been acknowledgment by all thyroid professional societies that there may be a role for the use of combination therapy; this represents a significant shift in the clinical practice guidelines. Further bolstering this trend are the recent findings that the Thr92AlaD2 polymorphism may reduce thyroid hormone signaling, resulting in localized and systemic hypothyroidism. This strengthens the hypothesis that treatment options could be personalized, taking into consideration genotypes and comorbidities. The development of long-acting formulations of liothyronine and continued advancements in development of thyroid regenerative therapy, may propel the field closer to adoption of a physiologic thyroid hormone replacement regimen with combination therapy.

Treatment of Hypothyroid Patients With L-Thyroxine (L-T4) Plus Triiodothyronine Sulfate (T3S). A Phase II, Open-Label, Single Center, Parallel Groups Study on Therapeutic Efficacy and Tolerability

Sodium salt of levothyroxine (L-T4) is the treatment of choice of hypothyroidism. Yet, L-T4 monotherapy produces supoptimal 3,5,3'-triiodothyronine (T3)/T4 ratio in serum, as compared to normal subjects, and a minority of hypothyroid individuals on L-T4 complain for an incomplete well-being. Orally administered 3,5,3'-triiodothyronine sulfate (T3S) can be converted to T3 in humans, resulting in steady-state serum T3 concentrations for up to 48 h. In this study (EudraCT number 2010-018663-42), 36 thyroidectomized hypothyroid patients receiving 100 (group A), 125 (group B), or 150 μg (group C) L-T4 were enrolled in a 75 days study in which 25 μg L-T4 were replaced by 40 μg of T3S. A significant, progressive reduction in mean FT4 values was observed, being the largest in the group A and the smallest in group C, while no relevant variations in FT3 and total T3 serum values were observed in the three groups. TSH serum levels increased in all groups, the highest value being observed in group A. Lipid parameters did not show clinically significant changes in all groups. No T3S-related changes in the safety laboratory tests were recorded. No adverse event was judged as related to experimental treatment, and no patient discontinued the treatment. Twelve patients judged the L-T4+T3S treatment better than L-T4 alone, while no patient reported a preference for L-T4 over the combined treatment. In conclusion, the results of this study indicate that a combination of L-T4+T3S in hypothyroid subjects may allow mainteinance of normal levels of serum T3, with restoration of a physiological FT4/FT3 ratio and no appearance of adverse events. Further studies are required to verify whether the LT4+T3S chronic combined treatment of hypothyroidism is able to produce additional benefits over L-T4 monotherapy.

Management of hypothyroidism with combination thyroxine (T4) and triiodothyronine (T3) hormone replacement in clinical practice: a review of suggested guidance

Background

Whilst trials of combination levothyroxine/liothyronine therapy versus levothyroxine monotherapy for thyroid hormone replacement have not shown any superiority, there remains a small subset of patients who do not feel well on monotherapy. Whilst current guidelines do not suggest routine use of combination therapy they do acknowledge a trial in such patients may be appropriate. It appears that use of combination therapy and dessicated thyroid extract is not uncommon but often being used by non-specialists and not adequately monitored. This review aims to provide practical advice on selecting patients, determining dose and monitoring of such a trial.

Main body

It is important to select the correct patient for a trial so as to not delay diagnosis or potentially worsen an undiagnosed condition. An appropriate starting dose may be calculated but accuracy is limited by available formulations and cost. Monitoring of thyroid function, benefits and adverse effects are vital in the trial setting given lack of evidence of safe long term use. Also important is that patients understand set up of the trial, potential risks involved and give consent.

Conclusion

Whilst evidence is lacking on whether a small group of patients may benefit from combination therapy a trial may be indicated in those who remain symptomatic despite adequate levothyroxine monotherapy. This should be undertaken by clinicians experienced in the field with appropriate monitoring for adverse outcomes in both short and long term.

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.

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.

Thyroid hormone replacement therapy: three 'simple' questions, complex answers

Current guidelines recommend that hypothyroid patients should be treated with levothyroxine, which in the vast majority of the cases leads to resolution of the symptoms and normalization of serum free T4 (FT4), T3 and TSH levels. However, a small group of hypothyroid patients remain symptomatic for neurocognitive dysfunction despite normal serum FT4 and TSH, which could be explained by localized brain hypothyroidism. More than half of the T3 in the brain is produced locally via the action of the type II deiodinase (D2) and variability/defects in this pathway could explain the residual symptoms. If this rationale is correct, adding liothyronine to the replacement therapy could prove beneficial. However, with a few exceptions, several clinical trials failed to identify any beneficial effects of combined therapy. More recently, the results of a large clinical trial revealed a better neurocognitive outcome with combined therapy only in hypothyroid patients carrying a polymorphism in the DIO2 gene. This obviously needs to be confirmed by other groups but it is tempting to speculate that combined levothyroxine and liothyronine has a place in the treatment of hypothyroidism, for some.

Longitudinal comparison of thyroxine pharmacokinetics between pregnant and nonpregnant women: a stable isotope study

The treatment of maternal hypothyroidism presents clinicians with a unique challenge, because dosing regimens previously developed and validated for nonpregnant women cannot be easily extrapolated to dosing in pregnancy. Thyroid hormone requirement increases by 20% to 40% early during pregnancy, persisting throughout gestation. Accordingly, women with treated hypothyroidism need to increase their levothyroxine dose to prevent maternal hypothyroidism and the associated impaired cognitive development and increased fetal mortality. We investigated the pharmacokinetic properties of levothyroxine during pregnancy through the use of a novel, traceable form of levothyroxine. The objective was to conduct a longitudinal study to determine whether levothyroxine pharmacokinetics differ in the pregnant versus nonpregnant state. We used a unique C-levothyroxine-tracer method to distinguish between endogenous and exogenous levothyroxine and studied the pharmacokinetics of a single oral dose of levothyroxine using tandem mass spectrometry. Moreover, we were able to detect single dose amounts of the drug, in picogram/mL concentrations. The area under the curve was 23.0 ng*h/mL in pregnancy and 14.8 ng*h/mL in nonpregnant women (P < 0.03) with median serum half-lives of 32.1 hours and 24.1 hours, respectively (P < 0.04). Further research involves the measurement of free thyroxine on these samples using tandem mass spectrometry. Future work should focus on the mechanisms responsible for the gestational differences in pharmacokinetics and whether these should necessitate dose schedule changes in pregnancy.

Thyroid function in malignant lymphoma

Thyroid function was studied in 36 patients with various stages of malignant lymphoma. Stage IVB patients exhibited characteristic changes in thyroid biochemistry in the form of lowered triiodothyronine (T3) and elevated free thyroxine (FT4), but normal thyroxine. Moreover, the concentration of thyroxine-binding prealbumin and albumin was lowered, whereas thyroxine-binding globulin was normal. Thyroid-stimulating hormone was slightly elevated but showed a normal increase after administration of thyrotrophin-releasing hormone. Patients with less extensive disease differed only slightly from the controls. The results agree with previous studies of patients suffering from other chronic diseases. The mechanisms underlying the hormonal changes have been only partially elucidated. When investigating patients with disseminated malignant disease for thyroid disease, the above mentioned changes in thyroid biochemistry must be borne in mind. Single analyses of FT4 and T3 may give rise to a false assumption of hyper- or hypothyroid states in patients who are in fact euthyroid.

Minor signs and symptoms of toxicity in a young woman in spite of massive thyroxine ingestion

The serum concentrations of thyroxine, 3,5,3'-triiodothyronine and 3,3',5'-triiodothyronine were followed during nine days in a case of acute thyroxine intoxication. On admission the concentrations were 11-12 times, 5-6 times and 16 times the normal mean, respectively. There was a striking discrepancy between the high concentrations of active hormones and the minor clinical symptoms.

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.

The liver in other (nondiabetic) endocrine disorders

The liver has a major role in the proper maintenance of intermediate metabolism and endocrine homeostasis. It contains enzymes that are essential for hormonal biotransformation and the regulation of numerous metabolic reactions, which control hormone metabolism. The liver also manufactures several proteins, which carry circulating hormones to their effector sites. The endocrine system exerts tight control of the metabolic reactions within the liver, which also can be disturbed by endocrine disorders. These types of interactions and the effects of the exogenous hormones and the drugs that are used as treatment for hormonal disorders are discussed.

Effects of supraphysiological doses of L-thyroxine on cognitive function in healthy individuals

Cognitive disturbance is commonly associated with disorders of the thyroid gland, particularly hypothyroidism, and usually subsides following thyroid hormone replacement therapy. In contrast, the effects of thyroid hormones on cognitive functions in healthy individuals have rarely been studied. The goal of this open-label study was to investigate the short-term effects (duration of administration 45 days on average) of supraphysiological doses of L-thyroxine (L-T(4)) on cognitive performance in young, euthyroid, healthy subjects. Eleven subjects performed a comprehensive neuropsychological test battery, once without and once during administration of supraphysiological doses of L-T(4). There were no significant differences in any of the cognitive test results between the two test sessions. The results of this study do not support our working hypothesis that thyroid hormone can change cognitive performance in young, euthyroid, healthy individuals.

Effects of supraphysiological thyroxine administration in healthy controls and patients with depressive disorders

BACKGROUND

Thyroxine (T(4)) in supraphysiological doses has been found to be an effective supplemental treatment in open studies for refractory mood disorders. Unexpectedly, only minimal side effects have been reported. The goal of the present study was to investigate whether healthy controls and depressed patients differ in their ability to tolerate supraphysiological doses of T(4).

METHODS

This was an 8-week open study to investigate side effects and levels of thyroid hormones in 13 healthy controls and to compare results with those of 13 patients with refractory depression (unipolar and bipolar) undergoing the similar procedures and T(4) dosing regimen in a previous augmentation study.

RESULTS

The rate of discontinuation due to side effects was significantly higher in the control group than for the patients (38% versus 0%). The severity of the side effects in the controls increased significantly during treatment with T(4). The side effect scores of the patients were higher than those of the controls prior to T(4) treatment, but did not change significantly during the treatment period. Although the serum concentrations of thyroid hormones rose significantly in both groups, concentrations of fT(3) and fT(4) were significantly higher in the controls.

CONCLUSIONS

Healthy controls and depressed patients respond significantly differently to supraphysiological T(4). Healthy controls experience higher elevations of thyroid hormones in response to supraphysiological T(4), thus inducing significantly more side effects and discontinuation.

LIMITATIONS

Open-label study; groups were studied at different times; in contrast to healthy controls, depressed patients were also taking antidepressants.

CLINICAL RELEVANCE

Studies provide safety and tolerability data on treatment with supraphysiological doses of T(4).

Serum thyroglobulin measurement. Utility in clinical practice

Serum thyroglobulin measurement has greatly facilitated the clinical management of patients with differentiated thyroid cancer and a variety of other thyroid disorders. Thyroglobulin autoantibodies remain a significant obstacle to the clinical use of thyroglobulin measurement. The interpretation of any given thyroglobulin value requires the careful synthesis of all pertinent clinical and laboratory data available to the clinician. The diagnostic use of rhTSH-stimulated thyroglobulin levels has greatly facilitated the follow-up of low-risk patients with thyroid cancer. Although the measurement of thyroglobulin mRNA from peripheral blood is likely to affect the future management of these patients, it is expected that serum thyroglobulin measurement will continue to have a principal role in the care of patients with differentiated thyroid cancer.

Thyrotoxicosis in children

Graves' disease is the predominant cause of hyperthyroidism in the pediatric age group. Other disorders must be recognized, however, because adequate management relies on a precise diagnosis. Careful monitoring of the thyroid status is required during this active phase of growth and development.

Multicompartmental analysis of triiodothyronine kinetics in hypothyroid patients treated orally or intravenously with triiodothyronine

Kinetic studies were performed with i.v. 125I T3 in four athyreotic women on two occasions each, once while they were taking oral T3 (30 micrograms T3 every 12 h) and again while on i.v. T3 replacement (same dosage schedule). The kinetic data were analyzed by a 7-compartment model, representing the plasma volume, the fast and slow peripheral exchange compartments, the iodide pool (as a delay compartment prior to appearance in the urine), the intestine (as a delay compartment before appearance in the feces), and the urine and feces. Modeling was done by the SAAM methodology. All data sets, and also the mean data treated as though they were data from a single subject, were fitted for the two limit solutions in which all metabolism was assumed to be in one or the other of the exchange compartments. The mean data set was also fitted to a solution in which limits were imposed on the excretion parameters and the partition of metabolism between the 2 peripheral exchange compartments was estimated. We found that steady-state parameters for removal of T3 from the circulation (the MCRs and DRs) were increased during the i.v. T3 replacement period compared with the oral replacement period, especially in the fast exchange compartment. Measured serum stable T3 levels (RIA) were lower in the i.v. than in the oral study, both at 8 and at 12 h after the most recent T3 dose. These values corresponded to similar differences in the circulating T3 levels projected from the model, although the T3 values projected from the model were greater than the measured T3 levels for unknown reasons.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiology of endocrine skin interrelations

Hormones influence the skin and play a role in normal biologic processes. Keratinocytes can convert and synthesize endocrine hormones. Endocrine dysregulation of the skin and abnormalities of endocrine functions of keratinocytes may produce abnormal changes in the skin. Knowledge of the cutaneous metabolism of thyroid hormones, steroids, peptide hormones, and vitamin A derivatives is being rapidly updated. Skin manifestations of endocrine disorders result from imbalance in feedback loops maintaining endocrine homeostasis. Define molecular mechanisms of hormonal action on target cells underlie functional agonism and antagonism of hormonal signals aimed at governing epidermal turnover. The molecular synergism between vitamin A and other hormones may explain the therapeutic efficiency of combining retinoids with other therapies.

Alterations in 3,3'5'-triiodothyronine metabolism in response to propylthiouracil, dexamethasone, and thyroxine administration in man

To elucidate the mechanisms involved in altering serum 3,3',5'-triiodothyronine (rT3) levels with absolute or relative low 3,5,3'-triiodothyronine (T3) states in man, agents capable of lowering circulating T3 levels were sequentially administered to six euthyroid subjects. These agents included propylthiouracil (PTU) (300 mg/6 h X 5 d), dexamethasone (DEX) (2 mg/6 h X 5 d), and thyroxine (T4) (3.0 mg load and 0.3 mg/d X 5 d). [125I] rT3 clearance rates and rT3 production rates were then determined. Increased serum rT3 levels and rT3/T4 values occurred with both PTU and DEX as compared with control, while T4 increased serum rT3 but did so without changing rT3/T4 values. The rT3 clearance rate was significantly decreased by PTU without altering production rate, while DEX increased the rT3 production rate without altering the rT3 clearance rate. T4 administration did not change rT3 clearance but proportionately increased rT3 production. These responses indicate that circulating rT3 predominantly originates from a non-PTU inhibitable deiodinase enzyme system located in extrahepatic tissues. This enzyme system appears to have a high capacity and low affinity for T4 and can be stimulated by DEX administration.

Replacement dose, metabolism, and bioavailability of levothyroxine in the treatment of hypothyroidism. Role of triiodothyronine in pituitary feedback in humans

A change in the formulation of the levothyroxine preparation Synthroid (Flint) in 1982 prompted us to reevaluate the replacement dose of this drug in 19 patients with hypothyroidism. The dose was titrated monthly until thyrotropin levels became normal. The mean replacement dose (+/- SD) was 112 +/- 19 micrograms per day, significantly less (P less than 0.001) than the dose of an earlier formulation--169 +/- 66 micrograms per day--used in a similar study (Stock JM, et al. N Engl J Med 1974; 290:529-33). The fractional gastrointestinal absorption of a tablet of the current formulation is 81 percent, considerably higher than the earlier estimate of 48 percent. Using high-performance liquid chromatographic analysis, we found that the current tablet contains the amount of thyroxine stated by the manufacturer. By measuring the bioavailability of the earlier type of tablet in five patients, we inferred that the strength of the previous tablet had been overestimated. In the present study, the thyrotropin levels of patients on replacement therapy returned to normal when serum triiodothyronine concentrations were not significantly different from those of controls (122 vs. 115 ng per deciliter [1.87 vs. 1.77 nmol per liter]), but when serum thyroxine levels were significantly above those of controls (11.3 vs. 8.7 micrograms per deciliter [145 vs. 112 nmol per liter], P less than 0.001). These findings suggest the possibility that in humans, serum triiodothyronine may play a more important part than serum thyroxine in regulating the serum thyrotropin concentration.

Serum concentration of unsaturated thyroxine-binding globulin in hyper- and hypothyroidism

The concentration of serum thyroxine (T4)-binding globulin (TBG) not binding T4 (unsaturated TBG, u-TBG) was determined in hyper- and hypothyroidism. u-TBG was expressed as the product of TBG concentration and the ratio of free TBG capacity to maximal TBG capacity as determined by reverse-flow electrophoresis. u-TBG concentration in normal sera (n = 40) was 15.5 +/- 2.3 mg/l (mean +/- SD), or 257 +/- 38 nmol/l for a molecular weight of TBG of 60 000 daltons. u-TBG levels were significantly lower in hyperthyroidism (7.1 +/- 2.3 mg/l, n = 16, P less than 0.001) and higher in hypothyroidism (21.7 +/- 5.0 mg/l, n = 22, P less than 0.001). Based on partial correlation analysis, u-TBG was inversely correlated to serum T4 (r = -0.586, P less than 0.001), but not correlated to triiodothyronine (T3) (r=-0.180, NS). There was a reciprocal correlation between u-TBG concentration and the T3 uptake value (r = 0.748, P less than 0.001). There was also a reciprocal correlation of u-TBG with both % free T4 (r = 0.425, P less than 0.001) and % free T3 (r = 0.377, P less than 0.001), when the data were subjected to partial correlation analysis. These results provide the values for u-TBG concentration in hyper- and hypothyroidism, and support the concept that the free fractions of serum thyroid hormones may be determined by the number of binding sites of the TBG molecule that are not saturated with T4 in hyper-and hypothyroidism.

Thyroid function in juvenile diabetes

This article reviews our current knowledge on the effects of diabetes mellitus on thyroid function at the level of the pituitary-thyroid-peripheral tissue axis and attempts to determine its clinical importance.

Mechanisms governing the relative proportions of thyroxine and 3,5,3'-triiodothyronine in thyroid secretion

In subjects with normal thyroid function only a minor part of firculating 3,5,3'-triiodothyronine (T3) originates directly from the thyroid; the majority is produced in the peripheral tissues by deiodination of thyroxine (T4). However, T3 of thyroidal origin constitutes a relatively high fraction of the total T3 produced in many patients with thyroid hyperfunction or hypofunction. Such a relatively high T3 content in the secretion of the thyroid could be caused by a low T4/T3 ratio in thyroglobulin. Severe iodine deficiency is a well-known inducer of a low T4/T3 ratio, but a low T4/T3 ratio can also be produced independent of the iodine content. This is seen in in vitro studies of thyroglobulin iodination when small amounts of DIT are added to the incubation mixture and in vivo in TSH-treated animals and in patients with Graves' disease. Another mechanism for high thyroidal secretion of T3 could be an enhanced fractional deiodination of T4 to T3 in the thyroid. In vitro thyroid perfusion studies have shown that the T3 content of thyroid secretions is higher than would be expected from the T4/T3 ratio of thyroid hydrolysate and that the major mechanism is deiodination of T4 to T3. Thyroxine deiodinases are also present in the human thyroid, and the amount of T4 deiodinase is enhanced in the thyroids from patients with medically treated Graves' disease and in the hyperstimulated thyroids of rats. Other factors of possible importance for the mixture of T3 and T4 secreted by the thyroid are a relatively faster liberation of T3 than of T4 from thyroglobulin during partial hydrolysis (this faster release of T3 is probably the mechanism behind the more "rapid" secretion of T3 than of T4), or some kind of thyroid heterogeneity leading to pinocytosis and hydrolysis of thyroglobulin with a lower T4/T3 ratio than that of average thyroglobulin.

Serum thyroid hormone concentrations and recovery of TSH secretion after excision of autonomously functioning thyroid nodules

Serum concentrations of TSH, TT4, TT3 and rT3 were monitored for one month after excision of ten autonomous thyroid adenomas which had suppressed TSH secretion. Basal serum TSH levels start to increase between 30 hr and 20 days after surgery reaching normal and steady levels by the 24 day. Serum TT3 concentrations rapidly decrease in the first 20 hr to values near the lower limit of the normal range. Thereafter TT3 levels change little until pituitary TSH secretion recovers. Serum TT4 levels also fall, but much more slowly than TT3. Rising TSH levels stimulate residual extranodular thyroid tissue secretion of both TT3 and TT4. However, serum TT3 levels rise more rapidly than TT4 levels. Preoperative serum concentrations of both TT4 and TT3 are related directly to the time required until the beginning of the postoperative rise in TSH levels, and inversely with the maximal postoperative serum TSH concentration achieved. Throughout the study period, for all patients, serum TSH concentrations were inversely related to serum TT4 concentrations. These data suggest that, although the time required until the onset of recovery of TSH secretion is directly related to preoperative levels of TT4 and TT3, the regulation of postoperative TSH levels is dependent upon serum TT4 levels. Also, the serum TT3 levels constant at the lower limit of the normal range before recovery of TSH secretion, and the preferential rise in serum TT3 concentrations associated with rising TSH secretion, may prevent or abbreviate temporary postoperative hypothyroidism.

The metabolism of T4 and T3 in cultured chick-embryo heart cells

Thyroxine (T4) and tri-iodothyronine (T3) were metabolized in cultured chick-embryo heart cells via inner-ring de-iodination and O-sulfation. The products of T3 metabolism were 3,3'T2, 3'T1, and sulfated esters of T3 and 3,3'T2. The major product of T4 degradation was 3,3',5'-T3 (rT3). ONo T3 was produced from T4. Propylthioracil inhibited the metabolism of T3. Pretreatment of cultures with T3 or T4 enhanced the metabolism of both hormones; actinomycin D and cycloheximide inhibited the stimulatory effect of T3. The stimulation by T3 was linearly related to the log of the concentration of T3 in cells grown in normal chick serum or in cells grown in dehormonized serum. These results suggest that thyroid hormones induced an increased synthesis of the enzymes involved in their metabolism and therefore may regulate their own disposal rate.

Regulation of the conversion of thyroxine to triiodothyronine in the perfused rat liver

This study was undertaken to determine what factors control the conversion of thyroxine (T(4)) to triiodothyronine (T(3)) in rat liver under conditions approximating those found in vivo. Conversion of T(4) to T(3) was studied in the isolated perfused rat liver, a preparation in which the cellular and structural integrity is maintained and that can perform most of the physiologic functions of the liver. The perfused liver readily extracted T(4) from perfusion medium and converted it to T(3). Production of T(3) by the perfused liver was a function of the size of the liver, the uptake of T(4) by the liver, and the presence of T(4)-5'-deiodinase activity. Production of T(3) was increased by increasing the uptake of T(4) by liver, which could be accomplished by increasing the liver size, by increasing the perfusate T(4) concentration, or by decreasing the perfusate albumin concentration. These changes occurred without altering the conversion of T(4) to T(3). The liver had a large capacity for extracting T(4) and for T(4)-5'-deiodination to T(3), which was not saturated at a T(4) concentration of 60 mug/dl. Production of T(3) was decreased by inhibiting hepatic T(4)-5'-deiodinase with propylthiouracil, which decreased T(3) production by decreasing the conversion of T(4) to T(3). Propylthiouracil did not alter hepatic T(4) uptake. Fasting resulted in a progressive decrease in hepatic T(4) uptake to 42% of control levels by the 3rd d of fasting; this was accompanied by a proportionate decrease in T(3) production. The rate of conversion of T(4) to T(3) did not change during fasting. When T(4) uptake in 2-d-fasted rat livers was raised to levels found in fed rats by increasing the perfusate T(4) concentration from 10 to 30 mug/dl, T(3) production returned to normal. Again, no change in the rate of conversion of T(4) to T(3) was observed. These results indicate that the decreased hepatic T(3) production during fasting primarily results from decreased hepatic uptake of T(4), rather than from changes in T(4)-5'-deiodinase activity. Thus, these studies have delineated a new mechanism that functions independently of enzyme quantity or activity whereby production of T(3) from T(4) is regulated.

Dietary-induced alterations in thyroid hormone metabolism during overnutrition

Diet-induced alterations in thyroid hormone concentrations have been found in studies of long-term (7 mo) overfeeding in man (the Vermont Study). In these studies of weight gain in normal weight volunteers, increased calories were required to maintain weight after gain over and above that predicted from their increased size. This was associated with increased concentrations of triiodothyronine (T3). No change in the caloric requirement to maintain weight or concentrations of T3 was found after long-term (3 mo) fat overfeeding. In studies of short-term overfeeding (3 wk) the serum concentrations of T3 and its metabolic clearance were increased, resulting in a marked increase in the production rate of T3 irrespective of the composition of the diet overfed (carbohydrate 29.6 +/- 2.1 to 54.0 +/- 3.3, fat 28.2 +/- 3.7 to 49.1 +/- 3.4, and protein 31.2 +/- 2.1 to 53.2 +/- 3.7 microgram/d per 70 kg). Thyroxine production was unaltered by overfeeding (93.7 +/- 6.5 vs. 89.2 +/- 4.9 microgram/d per 70 kg). It is still speculative whether these dietary-induced alterations in thyroid hormone metabolism are responsible for the simultaneously increased expenditure of energy in these subjects and therefore might represent an important physiological adaptation in times of caloric affluence. During the weight-maintenance phases of the long-term overfeeding studies, concentrations of T3 were increased when carbohydrate was isocalorically substituted for fat in the diet. In short-term studies the peripheral concentrations of T3 and reverse T3 found during fasting were mimicked in direction, if not in degree, with equal or hypocaloric diets restricted in carbohydrate were fed. It is apparent from these studies that the caloric content as well as the composition of the diet, specifically, the carbohydrate content, can be important factors in regulating the peripheral metabolism of thyroid hormones.

Longitudinal study or serum thyroid hormones, chorionic gonadotrophin and thyrotrophin during and after normal pregnancy

Measurements of serum levels of thyroxine (T4), free T4, 3,5,3'-triiodothyronine (T3), free T3, 3,3',5'-triiodothyronine (reverse T3, rT3), thyroxine-binding globulin capacity (TBGcap), chorionic gonadotrophin (hCG) and thyrotrophin (TSH) were carried out prospectively in eight women with uncomplicated pregnancies, in order to examine interrelationships between the thyroid gland and thyroid stimulating hormones during pregnancy. During pregnancy the levels of T4, free T4, T3, rT3 and TBGcap were significantly elevated, and TSH was decreased. It was noted that the elevation of T4 was maintained from the 8th to the 27th week of gestation while the level of TBGcap progressively increased. The levels of free T4 and rT3 in the first and third trimesters were significantly higher than those of age-matched, non-pregnant women. The levels of hCG showed a biphasic variation, with a peak in the 8th to 15th weeks, followed by a decline in the second trimester and a small, secondary elevation in the 32nd to 39th weeks. This later elevation was positively correlated with changes in free T4 and free T3 levels. The increase of serum T4 accompanied by an increase of free T4 in the first trimester appeared due to augmented secretion of T4, rather than being secondary to the elevated levels of TBGcap.

Diurnal rhythm of pituitary-thyroid axis in male rats and the effect of adrenalectomy

Levels of TSH, thyroxine (T4) and triiodothyronine (T3) were measured by radioimmunoassay in plasma (TSH, T4, T3) and pituitary (TSH) of 60-day old male Long-Evans rats. Definite diurnal rhythms were demonstrated in pituitary TSH, plasma TSH and T3 in intact rats, evidenced by the statistically significant differences between zenith and nadir for pituitary TSH, plasma TSH and T3. The zenith value of pituitary TSH and the nadir values of plasma TSH, T4 and T3 were observed at the same time (2400 h) as were the nadir value of pituitary TSH and the zenith values of plasma TSH, T4, and T3 (at 1200 h). Our results indicate that the rhythmicity of pituitary TSH content is a mirror image of that of plasma TSH. Adrenalectomy not only reduced plasma corticosterone levels to almost zero, but also decreased plasma T3 and T4 levels. In adrenalectomized rats, the absolute concentration of pituitary and plasma TSH increased at 1200 h and at 2400 h and, in both cases, the difference between values at 1200 h and at 2400 h persisted. The differences in plasma TSH, T3 and T4 between 1200 h and 2400 h were also observed in sham-adrenalectomized rats. These results suggest glucocorticoids do influence the pituitary-thyroid axis, but that the rhythmicity of the pituitary-thyroid axis does not seem to depend on the rhythmicity of glucocorticoid secretion.

Conversion of T3 and rT3 to 3,3'-T2: pH dependency

In the extrathyroidal deiodination of T4 the importance of T3 and rT3 for the peripheral action of thyroid hormones is well documented. With the development of a specific radioimmunoassay for 3,3'-T2, a deiodination product of both T3 and rT3, we were able to characterize these subsequent enzymatic reactions as well as the degradation of 3,3'-T2 in rat liver homogenate. It was found that the reaction T3 leads to 3,3'-T2 is slow compared to the conversion of T4 to T3. The pH activity profile shows a peak at 8.4. The reaction rT3 leads to 3,3'-T2 is very fast, with an apparent KM of 4 x 10(-7) M. The reaction velocity is significantly higher in acid than in alkaline pH. The deiodination of 3,3'-T2 is also faster in the acid than in the alkaline range. It is concluded that the outer ring of T4 is more readily deiodinated in acid and the inner ring in alkaline media, and that 3,3'-T2 is mainly degraded to 3-T1. In the acid pH range T3 may accumulate and in the alkaline range rT3 by the pH characteristics of these ractions. Therefore small shifts in pH can enhance the potent inhibitory action of rT3 on the 5'-deiodination of T4 adding another mechanism to the peripheral regulation of thyroid hormone activity.

L-triiodothyronine and L-reverse-triiodothyronine generation in the human polymorphonuclear leukocyte

Extrathyroidal monodeiodination of l-thyroxine (T(4)) is the principal source of l-triiodothyronine (T(3)) and l-reverse-triiodothyronine (rT(3)) production. To define some of the cellular factors involved, we examined T(3) and rT(3) generation from added nonradioactive T(4) in human polymorphonuclear leukocytes, using radioimmunoassays to quantify the T(3) and rT(3) generated. Under optimum incubation conditions which included a pH of 6.5 in sucrose-acetate buffer, the presence of dithiothreitol as a sulfhydryl-group protector, and incubation in an hypoxic atmosphere, significant net generation of T(3) and rT(3) was observed. Of the several subcellular fractions studied, the particulate fraction obtained by centrifugation at 27,000 g was found to possess the highest T(3)- and rT(3)-generating activities per unit quantity of protein. With respect to T(3) generation from substrate T(4), the K(m) was 5 muM and the V(max) was 7.2 pmol/min per mg protein. Propylthiouracil, methimazole, and prior induction of phagocytosis inhibited both T(3) and rT(3) generation, but T(3) generation was inhibited to a greater extent. rT(3), in a concentration equimolar to that of substrate T(4), did not alter T(3) generation, but inhibited T(3) generation when the molar ratio of rT(3) to T(4) approached 10:1. Under the incubation conditions employed, particulate fractions of leukocytes obtained from five cord blood samples displayed an essentially normal relationship between T(3)- and rT(3)-generating activities, despite the distinctly divergent serum T(3) and rT(3) concentrations in these samples. From our findings, we draw the following conclusions: (a) the human polymorphonuclear leukocyte possesses the ability to generate T(3) and rT(3) from substrate T(4); (b) the T(3)- and rT(3)-generating activities are associated principally with the 27,000 g particulate fraction and display enzymic characteristics with a sulfhydryl-group requirement; (c) T(3)-generating activity appears to be more susceptible to inhibitory influences than rT(3)-generating activity; and (d) in cord blood leukocytes, the putative enzymes catalyzing T(3) and rT(3) generation appear to be functionally intact under the experimental conditions employed.

Regulation of thyroid hormone metabolism in rat liver fractions

The nature of the conversion of thyroxine (T4) to triiodothyronine (T3) and reverse triiodothyronine (rT3) was investigated in rat liver homogenate and microsomes. A 6-fold rise of T3 and 2.5-fold rise of rT3 levels determined by specific radioimmunoassays was observed over 6 h after the addition of T4. An enzymic process is suggested that converts T4 to T3 and rT3. For T3 the optimal pH is 6 and for rT3, 9.5. The converting activity for both T3 and rT3 is temperature dependent and can be suppressed by heat, H2O2, merthiolate and by 5-propyl-2-thiouracil. rT3 and to a lesser degree iodide, were able to inhibit the production of T3 in a dose related fashion. Therefore the pH dependency, rT3 and iodide may regulate the availability of T3 or rT3 depending on the metabolic requirements of thyroid hormones.