Determination of iodothyronine absorption and conversion of L-thyroxine (T 4 ) to L-triiodothyronine (T 3 ) using turnover rate techniques

Adrenal response to ACTH challenge alters thyroid and immune function and varies with body reserves in molting adult female northern elephant seals

Intrinsic stressors associated with life-history stages may alter the responsiveness of the hypothalamic-pituitary-adrenal axis and responses to extrinsic stressors. We administered adrenocorticotropic hormone (ACTH) to 24 free-ranging adult female northern elephant seals (NESs) at two life-history stages: early and late in their molting period and measured a suite of endocrine, immune, and metabolite responses. Our objective was to evaluate the impact of extended, high-energy fasting on adrenal responsiveness. Animals were blood sampled every 30 min for 120 min post-ACTH injection, then blood was sampled 24 h later. In response to ACTH injection, cortisol levels increased 8- to 10-fold and remained highly elevated compared with baseline at 24 h. Aldosterone levels increased 6- to 9-fold before returning to baseline at 24 h. The magnitude of cortisol and aldosterone release were strongly associated, and both were greater after extended fasting. We observed an inverse relationship between fat mass and the magnitude of cortisol and aldosterone responses, suggesting that body reserves influenced adrenal responsiveness. Sustained elevation in cortisol was associated with alterations in thyroid hormones; both tT3 and tT4 concentrations were suppressed at 24 h, while rT3 increased. Immune cytokine IL-1β was also suppressed after 24 h of cortisol elevation, and numerous acute and sustained impacts on substrate metabolism were evident. Our data suggest that female NESs are more sensitive to stress after the molt fast and that acute stress events can have important impacts on metabolism and immune function. These findings highlight the importance of considering life-history context when assessing the impacts of anthropogenic stressors on wildlife.

Levothyroxine: Conventional and novel drug delivery formulations

Despite the fact that levothyroxine is one of the most prescribed medications in the world, its bioavailability has been reported to be impaired by many factors, including interfering drugs or foods and concomitant diseases, and persistent hypothyroidism with a high dose of levothyroxine is thus elicited. Persistent hypothyroidism can also be induced by noninterchangeability between formulations and poor compliance. To address these issues, some strategies have been developed. Novel formulations (liquid solutions and soft-gel capsules) have been designed to eliminate malabsorption. Some other delivery routes (injections, suppositories, sprays, and sublingual and transdermal administrations) are aimed at circumventing different difficulties in dosing, such as thyroid emergencies and dysphagia. Moreover, nanomaterials have been used to develop delivery systems for the sustained release of levothyroxine to improve patient compliance and reduce costs. Some delivery systems encapsulating nanoparticles show promising release profiles. In this review, we first summarize the medical conditions that interfere with the bioavailability of oral levothyroxine and discuss the underlying mechanisms and treatments. The efficacy of liquid solutions and soft-gel capsules are systematically evaluated. We further summarize the novel delivery routes for levothyroxine and their possible applications. Nanomaterials in the levothyroxine field are then discussed and compared based on their load and release profile. We hope the article provides novel insights into the drug delivery of levothyroxine.

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.

Positive Impact of Thermal Manipulation During Embryogenesis on Foie Gras Production in Mule Ducks

Animal studies have shown that very early life events may have programing effects on adult metabolism and health. In this study, we aim, for the first, time to elucidate the effects of embryonic thermal manipulation (TM) on the performance of overfed mule ducks, in particular for the production of foie gras (fatty liver). We designed three embryonic TMs with different protocols for increasing the incubation temperature during the second part of embryogenesis, to determine whether hepatic metabolism could be “programed” to improve its fattening response to overfeeding at the age of three months. Initial results confirm that an increase in the incubation temperature leads to faster development (observed for all treated groups compared to the control group), and a decrease in the body surface temperature at birth. Thereafter, in a very innovative way, we showed that the three TM conditions specifically increased liver weights, as well as liver lipid content after overfeeding compared to the non-TM control group. These results demonstrate that embryonic TM effectively “programs” the metabolic response to the challenge of force-feeding, resulting in increased hepatic steatosis. Finally, our goal of improving foie gras production has been achieved with three different embryonic thermal stimuli, demonstrating the high reproducibility of the method. However, this repeatability was also perceptible in the adverse effects observed on two groups treated with exactly the same cumulative temperature rise leading to a reduction in hatchability (75 and 76% vs. 82% in control), in addition to an increase in the melting rate after cooking. These results suggest that embryonic thermal programing could be an innovative and inexpensive technique for improving foie gras production, although the specific protocol (duration, level or period of temperature increase), remains to be elucidated in order to avoid adverse effects.

The Levothyroxine Absorption Test: A Four-Year Experience (2015-2018) at The Mayo Clinic

Background: Levothyroxine (LT4) is the mainstay of therapy for hypothyroidism. Yet, despite physician efforts at dose titration, some patients remain hypothyroid on LT4 doses in excess of weight-based calculations, a condition known as refractory hypothyroidism. The LT4 absorption test (LT4AT) has been proposed to have utility in these patients by enabling distinction of LT4 malabsorption from pseudomalabsorption, a condition of intentional nonadherence. Given its rare use in clinical practice, we reviewed our institution's experience with the LT4AT to assess its impact on management of refractory hypothyroidism. Methods: We reviewed the charts of 16 patients diagnosed with refractory hypothyroidism and who had completed the LT4AT between January 2015 to January 2019. The primary aim was to determine the utility of this test in distinguishing LT4 malabsorption from pseudomalabsorption. Secondary aims were to determine whether the results of this test impacted physicians' management decisions, as well as to report on clinical outcomes at follow-up. Our LT4AT is a six-hour test wherein patients receive a weight-based dose of LT4 followed by serial measurements of total thyroxine (TT4) and thyrotropin (TSH). Percentage absorption is calculated using the following formula, with normal absorption being ≥60%: [Formula: see text] Results: Percentage absorption was calculated in 13 of 16 patients due to lack of TT4 data for 3 patients. Absorption was impaired in one patient (% absorbed = 0), who had known causes of malabsorption. The remaining 12 patients had normal absorption by hour 4 of the test (% absorption 60-158) in conjunction with upward TT4 trends. Clinical follow-up ranged from 1 to 32 months (median 6.5 months), with 11 patients having follow-up data. Six of these had normal or suppressed TSH values at most recent follow-up, and four had improved but persistent TSH elevations. The one said patient with malabsorption improved with intravenous LT4. Conclusions: The LT4AT can provide valuable information for distinguishing malabsorption from pseudomalabsorption. Our findings support the combined use of calculated percentage absorptions with TT4 trends for at least a four-hour time frame when making determinations regarding absorption.

Thyroxine and treatment of hypothyroidism: seven decades of experience

Hypothyroidism is one of the most common endocrine disorders, affecting as much as 10% of the global population. There is a rich cultural milieu of treatment history and interventions dating as far back as 2 millennia. Chinese cretins were treated with sheep thyroid in the 6th century. In 1890, transplanted animal thyroid tissue resulted in a prompt clinical response in a myxedematous patient, and in 1891 injections of sheep thyroid were reported. One year later, the oral administration of fresh sheep thyroid glands was noted to be effective. Within a few years, the danger of over-dosage with extracts was recognized and dosing guidance indicated a low dose start and gradual increase as required based on symptoms. Orally ingested extracts became widespread and by 1914 thyroxine had been crystallized. In 1927, thyroxine, was synthesized as an acid, limiting oral absorption. Finally a sodium salt of thyroxine was introduced in 1949. These synthetic preparations were then made available for clinical use. Prior to 1970, extracts and combination therapy with synthetic LT4 and LT3 were standard replacement until the peripheral deiodinase-mediated T4 to T3 conversion documented the endogenous generation of T3 from LT4 in athyreotic subjects. This resulted in advocacy for patients previously treated with combinations and desiccated thyroid be transitioned to L-thyroxine monotherapy. The determination of the optimal dose has evolved such that now a general recommendation for replacement dosage of LT4 is 1.6-1.7 mcg/kg/day. Thyroid hormone extracts were established prior to the FDA's establishment in 1906, and when the Food, Drug, and Cosmetic act of 1938 enhanced the FDA's regulatory authority. In 1997, FDA declared LT4 products to be new drugs subject to regulation and quickly a pharmacokinetic process to determine interchangeability among approved LT4 products ensued. Differences in bioavailability of 12.5% or more may be considered therapeutically equivalent and therefore such products interchangeable. To assure refill to refill consistency, all levothyroxine sodium products now meet a 95-105% potency specification throughout their labeled shelf-lives. Seventy years after Kendall's great achievement in isolating thyroxine, we have thyroxine products with precise amounts of synthetic hormone that meet demanding regulations to assure high product quality, predictable bioavailability given its narrow therapeutic range, and now are left with potential variance in the therapeutic efficacy among different preparations.

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.

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.

The History and Future of Treatment of Hypothyroidism

Thyroid hormone replacement has been used for more than a century to treat hypothyroidism. Natural thyroid preparations (thyroid extract, desiccated thyroid, or thyroglobulin), which contain both thyroxine (T4) and triiodothyronine (T3), were the first pharmacologic treatments available and dominated the market for the better part of the 20th century. Dosages were adjusted to resolve symptoms and to normalize the basal metabolic rate and/or serum protein-bound iodine level, but thyrotoxic adverse effects were not uncommon. Two major developments in the 1970s led to a transition in clinical practice: 1) The development of the serum thyroid-stimulating hormone (TSH) radioimmunoassay led to the discovery that many patients were overtreated, resulting in a dramatic reduction in thyroid hormone replacement dosage, and 2) the identification of peripheral deiodinase-mediated T4-to-T3 conversion provided a physiologic means to justify l-thyroxine monotherapy, obviating concerns about inconsistencies with desiccated thyroid. Thereafter, l-thyroxine monotherapy at doses to normalize the serum TSH became the standard of care. Since then, a subgroup of thyroid hormone-treated patients with residual symptoms of hypothyroidism despite normalization of the serum TSH has been identified. This has brought into question the inability of l-thyroxine monotherapy to universally normalize serum T3 levels. New research suggests mechanisms for the inadequacies of l-thyroxine monotherapy and highlights the possible role for personalized medicine based on deiodinase polymorphisms. Understanding the historical events that affected clinical practice trends provides invaluable insight into formulation of an approach to help all patients achieve clinical and biochemical euthyroidism.

When thyroid hormone replacement is ineffective?

PURPOSE OF REVIEW

In this article, we will consider the failure of thyroid hormone replacement therapy to normalize serum thyroid stimulating hormone concentrations. We will review circumstances and causes for failures, discuss pertinent unpublished personal cases of didactical value, and provide practical suggestions for providers encountering patients with similar presentations.

RECENT FINDINGS

Recent data are available on the benefit of novel formulations of levothyroxine therapy on malabsorption.

SUMMARY

Most frequently, reasons for ineffectiveness are noncompliance, inappropriate administration of levothyroxine, gastrointestinal disorders, and drug interactions. The diagnostic work-up should include careful history to elucidate the potential reasons for the ineffective therapy.

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.

L-thyroxine treatment in primary hypothyroidism does not increase the content of free triiodothyronine in cerebrospinal fluid: a pilot study

The association between cerebrospinal fluid (CSF) and serum concentration of thyroid hormones and pituitary thyrotropin stimulating hormone (TSH) was studied in nine hypothyroid patients (HT) before and in seven after L-thyroxine treatment. With L-thyroxine, median free T4 increased 4-fold in serum (3.5 pmol/L vs 17.5 pmol/L) and 3-fold in CSF, (3.9 pmol/L vs 11.5 pmol/L). Correspondingly, total T3 in serum increased two-fold (0.9 nmol/L vs 2.2 nmol/L). Unexpectedly, free T3 concentration in CSF was similar (1.5 pmol/L vs.1.5 pmol/L) before and during treatment. In HT, TSH in serum correlated with TSH in CSF as did free T4 in serum and in CSF. During L-thyroxine, the correlation with TSH in serum and CSF remained. Likewise, the free T4 concentration in serum correlated with that in CSF. However, no correlation was found between T3 in serum and free T3 in CSF. It seems evident that free T4 in serum equilibrates with that in the CSF both in the HT and during L-thyroxine. Despite a two-fold increase in total serum T3, free T3 in CSF remained unchanged, which agrees with previous results in rats showing that T3 is less exchangeable between serum and CSF. Alternatively, an accelerated conversion of T4 to T3 might have maintained the concentration of T3, due to strongly increased levels of TSH found in the hypothyroid state. The notion that free T4 in serum reflects the CSF concentration of free T4 is consistent with previous reports from studies in animals.

Fecal triiodothyronine and thyroxine concentrations change in response to thyroid stimulation in Steller sea lions (Eumetopias jubatus)

Variation in concentrations of thyroid hormones shed in feces may help to identify physiological states of animals, but the efficacy of the technique needs to be validated for each species. We determined whether a known physiological alteration to thyroid hormone production was reflected in hormone concentrations in the feces of Steller sea lions (Eumetopias jubatus). We quantified variation of triiodothyronine (T3) and thyroxine (T4) concentrations in feces following two intramuscular injections of thyrotropin (thyroid-stimulating hormone, TSH) at 24h intervals in four captive female sea lions. We found fecal T3 concentrations increased 18-57% over concentrations measured in the baseline sample collected closest to the time of the first TSH injection (p=0.03) and 1-75% over the mean baseline concentration (p=0.12) for each animal of all samples collected prior to injections. Peak T3 concentrations were greater than the upper bound of the baseline 95% confidence interval for three animals. The peak T3 response occurred 48h post-injection in three animals and 71h in the fourth. Post-injection T4 concentrations did not differ between the baseline sample collected closest to the time of the first TSH injection (p=0.29) or the mean baseline concentration (p=0.23) for each animal. These results indicate that induced physiological alterations to circulating thyroid hormone concentrations can be adequately detected through analyses of fecal T3 concentrations and that the technique may provide a means of non-invasively detecting metabolic changes in Steller sea lions.

Ovine thyroid stimulating hormone (TSH) heterologously stimulates production of thyroid hormones from Chinese soft-shell turtle (Pelodiscus sinensis) and bullfrog (Rana catesbeiana and Rana rugulosa) thyroids in vitro

Thyroid hormones are important for regulating a variety of developmental processes in vertebrates, including growth, differentiation, metamorphosis, and oxidative metabolism. In particular, this study focused on the in vitro production of thyroxine (T(4)) and triiodothyronine (T(3)) from thyroids in American bullfrogs (Rana catesbeiana), Chinese bullfrogs (Rana rugulosa Wiegmann), and Chinese soft-shell turtles (Pelodiscus sinensis) treated with ovine thyroid stimulating hormone (TSH) at different culture intervals (2, 4, 8, and 12 h) and dosages (1, 10, 50 or 100 ng). The levels of T(4) and T(3) in the tested animals were elevated upon stimulation in a time- and dose-dependent manner, indicating de novo synthesis of T(4) and T(3). Significantly higher hormone levels were observed in the Chinese bullfrog compared to the other two species, for both the time-course and dose-response experiments. Although the bullfrog secreted significantly higher levels of T(4) and T(3), a higher T(4)-conversion capacity was found in the Chinese soft-shell turtle. The highest ratios of T(3) to T(4) were observed in the American bullfrog and Chinese soft-shell turtle for the time-course and dose-response experiments, respectively. These findings suggest that the Chinese soft-shell turtle and bullfrog thyroids can accept ovine TSH for T(4)- and T(3)-formation in a time- and dose-dependent manner, supporting the hypothesis that the binding interactions between TSHs and thyroidal receptors are conserved in vertebrates.

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.

Studies on replacement and suppressive dosages of 1-thyroxine

Serum thyrotropin (TSH) levels were studied in 55 hypothyroid patients in order to determine adequate replacement and suppression dosages of 1-thyroxine (T4). In accordance with previous reports it was found that most patients had normal TSH levels and were clinically euthyroid at daily doses of 0.10-0.15 mg T4. None of the patients required a dose exceeding 0.20 mg. When the TSH levels normalized, serum thyroxine and serum triiodothyronine also fell to levels within their normal ranges. The effectiveness of various doses of T4 in suppressing the temporary rise in serum TSH concentration normally induced by thyrotropin-releasing hormone was examined in 57 patients treated with T4 for atoxic goitre or after subtotal surgical removal of such a goitre. The rise in TSH was not usually inhibited by a T4 dose of less than 0.20 mg, a finding which at least theoretically has implications for the adequate suppressive dose of T4.

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.

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.

Comparison of the effects of thyroxine and triiodothyronine on protein turnover and apoptosis in primary chick muscle cell cultures

Primary chick muscle cells were treated with physiological level of thyroxine (T4) or triiodothyronine (T3) to examine the effects of the hormones on growth, protein turnover, and apoptosis of the cells. Creatine kinase activity, as an index of differentiation, was increased by both T4 and T3. Even when the conversion from T4 to T3 was blocked by iopanoic acid, T4 increased creatine kinase activity. The rate of protein degradation estimated from [3H] tyrosine release was increased by T3 but not by T4. DNA cleavage and fragmentation, as indices of apoptosis, were induced by T3 but not by T4. These results show that T4 stimulates cell differentiation but not protein degradation and apoptosis in primary chick muscle cells, while all events are stimulated by T3.

Thyroid hormones and the treatment of depression: an examination of basic hormonal actions in the mature mammalian brain

Numerous clinical reports indicate that thyroid hormones can influence mood, and a change in thyroid status is an important correlate of depression. Moreover, thyroid hormones have been shown to be effective as adjuncts for traditional antidepressant medications in treatment-resistant patients. In spite of a large clinical literature, little is known about the mechanism by which thyroid hormones elevate mood. The lack of mechanistic insight reflects, in large part, a longstanding bias that the mature mammalian central nervous system is not an important target site for thyroid hormones. Biochemical, physiological, and behavioral evidence is reviewed that provides a clear picture of their importance for neuronal function. This paper offers the hypothesis that the thyroid hormones influence affective state via postreceptor mechanisms that facilitate signal transduction pathways in the adult mammalian brain. This influence is generalizable to widely recognized targets of antidepressant therapies such as noradrenergic and serotonergic neurotransmission.

Role of L-thyroxine in nuclear thyroid hormone receptor occupancy and growth hormone production in cultured GC cells

The contribution of L-thyroxine (T4) to nuclear thyroid receptor occupancy was studied in GC cells incubated with concentrations of 3,5,3'-triiodo-L-thyronine (T3) and T4 that resulted in free iodothyronine levels similar to those in serum of euthyroid rats. T4 accounted for 5.4-10% of the occupied receptors: T3 derived from T4 [T3(T4)] and T3 added to medium accounted for the remainder of receptor occupancy. Incubation with increasing medium free T4 resulted in a progressive increase in the contribution of T4 and T3(T4) to receptor occupancy. In incubations with 3.6-fold increased medium free T4, T4 accounted for 20.4%, and T3(T4) for 40.3% of receptor occupancy. These occupancy data and the experimentally determined Ka of thyroid receptor for T3 and T4 allowed calculation of nuclear free iodothyronine concentrations. Nuclear free T3 was 3-6-fold greater than medium free T3 and nuclear [corrected] free T4 was 12-19-fold greater than medium free T4. When GC cells were incubated with decreased medium free T3 and physiological medium free T4, both nuclear receptor occupancy and growth hormone production decreased as well. However, a twofold increase in medium free T4, in the presence of decreased medium free T3, restored receptor occupancy and growth hormone production to or near control values. These findings establish a role for T4 in addition to T3(T4) in nuclear receptor occupancy and biological activity in rat anterior pituitary tissue both in physiologic conditions and when medium free T4 is raised. The findings may have relevance to the sick euthyroid thyroid syndrome in which free T4 may be increased in some patients who have decreased serum free T3.

Utilization rates of thyroid hormones in mammals

1. Thyroxine utilization rates (T4U: N = 37 species) and triiodothyronine utilization rates (T3U: N = 7 species) scale with body mass to the 0.81 and 0.74 power respectively. 2. T4U rates tend to be lower in summer relative to other seasons, vary unpredictably during pregnancy and lactation, increase with regular physical activity, and generally decrease with age. 3. Both T4U and T3U increase with cold exposure, decrease with heat exposure and during fasting, and increase/decrease with hyperthyroidism/hypothyroidism. 4. Since these T4U and T3U changes are qualitatively similar but quantitatively different, the T3U/T4U ratio varies, suggesting a variable deiodination rate from thyroxine to triiodothyronine.

Thyroidal and peripheral production of 3,5,3'-triiodothyronine in humans by multicompartmental analysis

Multicompartmental analysis of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) kinetics based on the plasma disappearance curves of the two tracer hormones (J. J. DiStefano III, M. Jang, T. K. Malone, and M. Broutman. Endocrinology 110: 198-213, 1982 and J. J. DiStefano III, T. K. Malone, and M. Jang. Endocrinology 111: 108-117, 1982) was extended to include additional experimental data, namely, the appearance curve in plasma of labeled T3 generated in vivo from precursor T4. Kinetic analysis of data obtained in 14 studies carried out in normal subjects by using a composite six-pool model made it possible to quantify the contributions of the thyroid (3.3 micrograms.day-1.m-2) and the periphery (12.7 micrograms.day-1.m-2) to T3 production. T4 monodeiodination occurred mainly in peripheral tissues rapidly exchanging with plasma (10.7 micrograms T3.day-1.m-2), whereas only 2.0 micrograms T3.day-1.m-2 arose in slowly exchanging tissues. In contrast, if plasma disappearance curves only were analyzed, a value of 10.9 micrograms T3.day-1.m-2 was calculated for peripheral conversion in slowly exchanging tissues; this underscores the need for additional data, such as the [125I]T3 plasma appearance curve for the partition of central and peripheral production of T3.

Thyroid hormone and the gut

The gastrointestinal tract interacts actively with the thyroid hormones, T4 and T3. Both T4 and T3 are absorbed well but incompletely from the gut, and many factors affect this absorption. The mechanism of absorption is unknown. It is decreased in most malabsorption conditions, but is increased in the postgastrojejunotomy syndrome. It may involve conjugation to the glucuronide forms (T4G and T3G) in the mucosal cell with subsequent deconjugation prior to appearance in the portal vein blood. Absorption appears to be reduced in the presence of excess T4, and increased in hypothyroidism. The liver takes up a large fraction of the T4 and T3 from its circulation and returns a portion of the portal hormone back to the gut via the bile. There is also direct T4 and T3 secretion into the gut from the mesenteric circulation. Recent studies suggest that the gut plays a major role as a reservoir for the thyroid hormones, especially for T3, and that it may also play a role in the regulation of hormone activity.

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 iodothyronine concentrations during introduction of thyroxine replacement therapy in hypothyroidism

Serum concentrations of total and free T4 (TT4 and FT4), total and free T3 (TT3 and FT3), rT3, T4 binding globulin (TBG), T3 uptake (T3U) and TSH were measured in 12 patients with severe hypothyroidism before and during the introduction of replacement therapy with oral T4. The dose of T4 was increased by increments of 50 micrograms at intervals of 4 weeks to a total of 200 micrograms daily. There was a linear correlation between the concentrations of FT3 and FT4 (FT3 = 1.35 + 0.23FT4, r = 0.916, P less than 0.001). The correlation between TT3 and TT4 was more complex: the data were best fitted by the expression TT3 = 0.195 square root TT4, (r = 0.936, P less than 0.001). The relatively greater rise in TT3 initially may reflect a greater binding of T3 by TBG when the concentration of T4 is low. TBG concentration fell after 50 and 100 micrograms of T4 but did not change at the higher doses. There was a simple linear relation between TT4 and rT3 (rT3 = -0.022 + 0.0027TT4, r = 0.921, P less than 0.001). The expected inverse relation between TSH concentration and the thyroid hormones was seen, the three closest correlations being between the logarithm of the TSH concentration and FT3, the ratio T4/TBG and FT4 (r = 0.927, -0.917 and -0.900 respectively). These correlations were significantly better (P less than 0.05) than the correlations with untransformed TSH values. Suppression of TSH occurred while FT3 tended to remain within normal limits, but FT4 was often raised.(ABSTRACT TRUNCATED AT 250 WORDS)

[Acute factitious hyperthyroidism--moderate clinical symptoms in 3 cases under beta-blocker treatment]

The clinical and laboratory findings are described in three patients who ingested large amounts of L-thyroxine (two cases) and L-thyroxine together with L-triiodothyronine and who were treated with propranolol. Serum concentrations of thyroxine (maximum values 75 micrograms/dl, 64 micrograms/dl, and 20 micrograms/dl, respectively; normal range 4-12 micrograms/dl), triiodothyronine (maximum values 837 ng/dl, 453 ng/dl, and 566 ng/dl, resp.; normal range 80-180 ng/dl), reverse triiodothyronine (maximum values 235 ng/dl, 190 ng/dl, and 65 ng/dl, resp.; normal range 10-40 ng/dl) as well as free thyroxine equivalent and free triiodothyronine equivalent were monitored daily until they reached the normal range. Statistical analysis of the kinetics of these parameters indicated that the extreme thyroxine conversion was directed toward reverse triiodothyronine, partly due to the treatment with the beta-adrenergic blocker propranolol. The striking discrepancy between the high concentrations of the active hormones and the moderate clinical symptoms was most likely caused by peripheral effects of propranolol.

Interrelationships among thyroxine, growth hormone, and the sympathetic nervous system in the regulation of 5'-iodothyronine deiodinase in rat brown adipose tissue

Thyroxine (T4) and reverse triiodothyronine are potent inhibitors of brown adipose T4 5'-deiodinase (BAT 5'D). This effect does not require protein synthesis and is due to an acceleration of the rate of disappearance of the enzyme. Growth hormone (GH) also inhibits BAT 5'D but by a mechanism mediated through a long-lived messenger that correlates with growth rate. This explains the failure of BAT 5'D to increase abruptly after thyroidectomy as does the type II 5'-deiodinase in pituitary and central nervous system or the BAT 5'D itself after hypophysectomy. Although virtually inactive when given acutely, triiodothyronine replacement partially reduces BAT 5'D in hypophysectomized and thyroidectomized (Tx) animals probably as a result of improvement of systemic hypothyroidism and an increase in GH levels in the Tx rats. The fine balance between these inhibitory factors and the stimulatory effects of the sympathetic nervous system suggests an important physiologic role for the enzyme in this tissue.

Potential of brown adipose tissue type II thyroxine 5'-deiodinase as a local and systemic source of triiodothyronine in rats

Previous reports suggest that a type II iodothyronine 5'-deiodinase may become the main enzymatic pathway for extrathyroidal triiodothyronine (T3) generation when the enzyme levels are sufficiently elevated and/or liver and kidney type I 5'-deiodinase activity is depressed. The present studies assessed the potential of brown adipose tissue (BAT) type II 5'-deiodinase to generate T3 for the plasma pool. BAT 5'-deiodination (BAT 5'D) was stimulated by either short- (4 h) or long-term (7 wk) cold exposure (4 degrees C). Long-term cold exposure increased thyroxine (T4) secretion 40-60% and extrathyroidal T3 production three-fold. In cold-adapted rats treated with propylthiouracil (PTU), extrathyroidal T3 production was 10-fold higher than in PTU-treated rats maintained at room temperature. Cold did not stimulate liver or kidney 5'D, but the cold-adapted rats showed a six- to eightfold higher BAT 5'D content. PTU caused greater than 95% inhibition of liver and kidney 5'D, but did not affect BAT 5'D. Thyroidectomized rats maintained on 0.8 micrograms of T4/100 g of body weight (BW) per day were acutely exposed to 4 degrees C. In rats given 10 mg of PTU/100 g of BW, 4 h of cold exposure still caused a 12-fold increase in BAT 5'D, a 2.3-fold increase in plasma T3 production, and a 4.8-fold increment in the locally produced T3 in BAT itself. All these responses were abolished by pretreatment with the alpha 1-antiadrenergic drug prazosin. Regardless of the ambient temperature, liver 5'D activity was greater than 90% inhibited by PTU. These results indicate that BAT can be a major source of plasma T3 under suitable circumstances such as acute or chronic exposure to cold. Furthermore, BAT 5'D activity affects BAT T3 content itself, suggesting that thyroid hormone may have a previously unrecognized role in augmenting the thermogenic response of this tissue to sympathetic stimulation. Such interactions may be especially important during the early neonatal period in humans, a time of marked thermogenic stress.

Protective adaptation of low serum triiodothyronine in patients with chronic renal failure

Low serum triiodothyronine (T3) concentration is frequently found in patients with chronic renal failure (CRF). To test the hypothesis that this may serve to minimize protein catabolism in these patients, we measured nitrogen balance (Nb) in seven CRF and four control subjects in the basal state and when serum T3 concentration was elevated by L-triiodothyronine (LT3) and suppressed by sodium ipodate administration. In the basal state, both the controls and the CRF patients were in positive Nb, 0.02 +/- 0.51 and 0.58 +/- 0.34 g/day, respectively. During LT3 administration, Nb decreased to -0.80 +/- 0.39 g/day in the CRF patients (P less than 0.01), but remained positive, 0.22 +/- 0.67 g/day, in the controls. There was a significant negative correlation between serum T3 concentration and Nb in the CRF patients (r = -0.63, P less than 0.005), but not in the controls. Furthermore, urea nitrogen generation rate, calculated from urea kinetics, increased from a baseline of 4.6 +/- 0.55 to 6.0 +/- 0.50 mg/min during LT3 administration in the CRF patients (P less than 0.01). Sodium ipodate, which significantly lowered serum T3 concentrations, had little effect on nitrogen metabolism in the controls and the CRF patients. These data support the concept that low serum T3 concentrations may confer a protective effect on CRF patients regarding protein-nitrogen conservation and provide a rationale for not correcting such deficiency.

The binding of thyroid hormone receptors to DNA

The behaviour of tri-iodothyronine (T3)- and thyroxine (T4)-receptor complexes when bound to native DNA-cellulose is reported. Equal and large proportions of both T3- and T4-receptor complexes bind to DNA but although T3-receptor complexes are 99% recoverable by 0.5 M NaCl buffer elution, only 60-70% of the T4-receptor complexes are regained. The balance appears as free T4, apparently released as the T4-receptor complexes bind to the DNA whilst the corresponding receptor remains bound. This effect is independent of T4-receptor complex/DNA ratio up to ca. 4 fmol/micrograms DNA, of the presence of an equal amount of unoccupied receptor and of an eight-fold concentration range of both T4-receptor complex and DNA at a fixed ratio, in the cellulose matrix. Pre-formed receptor-DNA material, likewise, only accepts some 60% of the expected quantity of T4 whereas the capacity for T3 appears to be similar to that of free receptors.

Production rates and turnover of triiodothyronine in rat-developing cerebral cortex and cerebellum. Responses to hypothyroidism

Local 5'-deiodination of serum thyroxine (T4) is the main source of triiodothyronine (T3) for the brain. Since we noted in previous studies that the cerebral cortex of neonatal rats tolerated marked reductions in serum T4 without biochemical hypothyroidism, we examined the in vivo T4 and T3 metabolism in that tissue and in the cerebellum of euthyroid and hypothyroid 2-wk-old rats. We also assessed the contribution of enhanced tissue T4 to T3 conversion and decreased T3 removal from the tissues to the T3 homeostasis in hypothyroid brain. Congenital and neonatal hypothyroidism was induced by adding methimazole to the drinking water. Serum, cerebral cortex (Cx), cerebellum (Cm), liver (L) and kidney (R) concentrations of 125I-T4, 125I-T3(T4), and 131I-T3 were measured at various times after injecting 125I-T4 and 131I-T3. The rate of T3 removal from the tissues was measured after injecting an excess of anti-T3-antibody to rats previously injected with tracer T3. In euthyroid rats, fractional turnover rates of T3 per hour were: Cx, 0.26 +/- 0.02 (SE); Cm, 0.20 +/- 0.02; L, 0.98 +/- 0.07; R, 0.97 +/- 0.12; and the calculated unidirectional plasma T3 clearance by these tissues were, in milliliters per gram per hour: Cx = 0.38, Cm = 0.32, L = 5.0, and R = 5.6. In hypothyroidism, the fractional removal rates and clearances were reduced in all tissues, in cortex and cerebellum by 70%, and in liver and kidney ranging from 30 to 50%. While greater than 80% of the 125I-T3(T4) in the brain tissues of euthyroid rats was locally produced, in hypothyroid cerebral cortex and cerebellum the integrated concentrations of 125I-T3(T4) were 2.7- and 1.5-fold greater than in euthyroid rats. In the Cx, this response resulted from an approximately sixfold increase in fractional conversion and an approximately fourfold decrease in T3 removal rate hampered by a decreased uptake of T4 from plasma, whereas in Cm the response resulted only from the reduced T3 removal rate. In euthyroid rats, the calculated production rate of T3 in nanograms per gram per hour by the Cx was 0.96 and 0.89 by the Cm, which on a per organ basis equals 15 and 2%, respectively, of the extrathyroidal production rate as assessed in the body pool exchanging with plasma. Several conclusions can be drawn: Production of T3 by developing brain is a very active process in agreement with the need of thyroid hormones during this period. (b) The brain-plasma exchange of T3 is slow compared with that of L or R. (c) This, along with the active local production, explains the predominant role of the latter as a source of T3 for the brain. (d) In hypothyroidism, the Cx is protected by an increase in the efficiency of T4 to T3 conversion and a prolong residence time of T3 in the tissue, whereas the Cm is protected only by the latter. Because of the large fraction of the T3 produced locally and the active turnover rate of T3 in the brain, reductions in T3 removal rate are of utmost importance for T3 homeostasis in these tissues.

Development of a gas chromatographic selected ion monitoring assay for thyroxine (T4) in human serum

The components of a gas chromatographic mass spectrometric-selected ion monitoring (SIM) assay for thyroxine (T4) in human serum are described. The internal standard for the assay was synthesized from deuterium-labelled 3,5-diiodotyrosine and 3,5-diiodo-4-hydroxyphenylpyruvic acid. A novel method was developed for isolating the products of the coupling reaction. The results obtained by gas chromatography mass spectrometry SIM were compared with those of radioimmunoassay. The gas chromatographic mass spectrometric SIM assay would form the basis of a reference assay for T4.

Maintenance requirements of L-thyroxine in the treatment of hypothyroidism

By analyzing data from 68 hypothyroid patients ranging in age from 15 to 75 years who had been maintained in a euthyroid state for at least a year with oral levothyroxine sodium therapy, we attempted to determine whether there was a correlation between L-thyroxine dose and body weight or patient age. The mean replacement dose of L-thyroxine was 186 mug a day +/-69.6 or 2.76 mug per kg of body weight a day +/-0.82. There was a significant correlation between L-thyroxine dose and body weight (P<.001), but due to the small number of patients studied who were older than 65 years of age, no correlation was noted between L-thyroxine dose and age.

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.

Qualitative and quantitative differences in the pathways of extrathyroidal triiodothyronine generation between euthyroid and hypothyroid rats

Propylthiouracil (PTU) in maximally inhibitory doses for liver and kidney iodothyronine 5'-deiodinase activity (5'D-I), reduces extrathyroidal T4 to T3 conversion by only 60-70% in euthyroid rats. A second pathway of T4 to T3 conversion (5'D-II) has been found in pituitary, central nervous system, and brown adipose tissue. 5'D-II is insensitive to PTU and increases in hypothyroidism, whereas 5'D-I decreases in hypothyroid rats. Thyroxine (T4) and triiodothyronine (T3) kinetics were assessed in euthyroid and thyroidectomized rats by noncompartmental analysis after injecting [125I]T4 and [131I]T3. Neither the volume of distribution nor the rate of fractional removal of plasma T4 was affected by the thyroid status, but the fractional removal rate of T3 was approximately 50% reduced in hypothyroid rats (P less than 0.001). Fractional T4 to T3 conversion was 22% in euthyroid and 26% in hypothyroid rats. In euthyroid rats, sufficient PTU to inhibit liver and kidney 5'D-I greater than 90% reduced serum [125I]T3 after [125I]T4 (results given as percent dose per milliliter X 10(-3) +/- SEM): 4 h, control 16 +/- 2 vs. PTU 4 +/- 1, P less than 0.005, and 22 h, control 6.4 +/- 0.4 vs. PTU 3.6 +/- 0.7, P less than 0.025. In thyroidectomized rats, the same dose of PTU also inhibited 5'D-I in liver and kidney, but had no effect on the generation of serum [125I]T3 from [125I]T4. Similarly, after 1 microgram T4/100 g bw was given to thyroidectomized rats, serum T3 (radioimmunoassay) increased by 0.30 +/- 0.6 ng/ml in controls and 0.31 +/- 0.09 ng/ml in PTU-treated rats. However, when the dose of T4 was increased to 2-10 micrograms/100 g bw, PTU pretreatment significantly reduced the increment in serum T3. T3 clearance was not affected by PTU in hypothyroid rats. The 5'D-II in brain, pituitary, and brown adipose tissue was reduced to less than or equal to 60% of control by 30 micrograms/100 g bw reverse T3 (rT3), an effect that lasted for at least 3 h after rT3 had been cleared. In rT3-pretreated thyroidectomized rats, the generation of [125I]T3 from tracer [125I]T4 was reduced in the serum: 6 +/- 1 vs. 12 +/- 1 X 10(-3)% dose/ml, P less than 0.01, during this 3-h period. We conclude that virtually all the T3 produced from low doses of exogenous T4 given to hypothyroid rats is generated via a PTU-insensitive pathway, presumably catalyzed by the 5'D-II. This is a consequence of the enhanced activity of this low Km enzyme together with the concomitant decrease in the hepatic and renal 5'D-I characteristic of the hypothyroid state. The results indicate that in some circumstances, 5D-II activity may contribute to the extracellular, as well as intracellular, T3 pool.

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.

Total and free thyroid hormone concentrations in patients receiving maintenance replacement treatment with thyroxine

Total and free serum concentrations of thyroxine and triiodothyronine were measured in 122 subjects with hypothyroidism who were clinically well while receiving conventional replacement treatment with thyroxine. In a third of patients concentrations of total and free thyroxine were raised, often considerably; nevertheless concentrations of total and free triiodothyronine were usually normal. Though significant correlations were obtained between total triiodothyronine concentrations and total thyroxine concentrations (p less than 0.001) and between the triiodothyronine concentrations and free thyroxine concentrations (p less than 0.001) the slope of the line of the regression equation describing these correlations was small, hence large increases in both total and free thyroxine concentrations were accompanied by only modest increases in total and free triiodothyronine concentrations. The presence of total or free thyroxine concentrations above normal in patients taking thyroxine therefore are not necessarily of clinical consequence. In the assessment of adequacy of replacement treatment with thyroxine the most logical combination of in vitro thyroid function test results may be a normal thyrotrophin concentration and normal free triiodothyronine concentration.

Peripheral tissue mechanism for maintenance of serum triiodothyronine values in a thyroxine-deficient state in man

The present study was undertaken to define the source of endogenous triiodothyronine (T3) production responsible for maintaining serum T3 levels in euthyroid subjects with depressed serum thyroxine (T4) values. After withdrawal from 4 wk of exogenous T3 administration, a 22% decline in serum T3 values (from 129 +/- 6 to 99 +/- 4 ng/dl) was observed in six euthyroid subjects, despite a twofold reduction in serum T4 concentrations (from 7.5 +/- 0.5 to 3.2 +/- 0.5 micrograms/dl). This was accompanied by a nearly twofold increase in serum T3/T4 ratio values (17 +/- 1 to 29 +/- 6) but no significant alteration in reverse T3/T4 ratio values. This phenomenon did not appear to be thyroid stimulating hormone (TSH) dependent, since base-line serum TSH values were subnormal. Nor was it dependent on changes in thyroid gland function, since a blunted T3 response to exogenous bovine TSH occurred and pharmacologic doses of iodide did not influence the phenomenon. The finding in three athyreotic subjects that serum T3/T4 ratio values increased from 14 +/- 1 on T4 therapy (mean serum T4, 9.6 +/- 0.8 micrograms/dl and T3, 132 +/- 8 ng/dl) to 40 +/- 2 after withdrawal from 2 wk of T3 administration (serum T4 1.2 +/- 0.1 micrograms/dl and T3 46 +/- 3 ng/dl) provided direct evidence that an alteration in peripheral thyroid hormone metabolism was probably responsible for these findings previously observed in euthyroid subjects. The results of this study support the possible existence in euthyroid man of a peripheral tissue autoregulatory mechanism for maintaining serum T3 values in states of T4 deficiency. Whether this process involves an alteration in the efficiency of T4 to T3 conversion or the rate of T3 clearance is presently unknown.

Aging and the thyroid. Decreased requirement for thyroid hormone in older hypothyroid patients

In 84 patients, aged 23 to 84, with primary hypothyroidism, the daily dose of thyroxine needed to lower the serum thyrotropin level into the normal range was significantly less in older patients than in younger ones (p less than 0.01). Most of the difference between middle-aged (40 to 60 years) and older patients (greater than 60 years) was due to a decrease in the required dose in men; there was no difference in the dose needed by women in these age groups. Previous hyperthyroidism did not affect the dose of thyroxine required; it is unlikely that residual autonomous thyroid tissue affected the dose. Although the wide range of doses needed precludes use of these data in calculating a dose of thyroxine for an individual patient, doses of 100 micrograms per day or less were common in patients over age 40, and a few patients over age 60 needed 50 micrograms per day or less. Thus, (1) there is a sound physiologic basis for the common practice of using low doses of thyroxine, e.g., 25 micrograms per day, as initial therapy in older hypothyroid patients and (2) it may be reasonable to reassess the dose of thyroxine after several years in older patients.

Bioavailability of thyroid hormones from oral replacement preparations

We evaluated gastrointestinal absorption in normal subjects of T4 and T3 from synthetic T3 tablets (Cytomel, SKF), desiccated thyroid tablets (Armour), thyroglobulin tablets (Proloid, Warner-Chilcott) and synthetic L-T4 tablets (Synthroid, Flint and Levothroid, Armour). Measurements of serum T4 and T3 concentrations and free hormone indices were made at multiple times after tablet ingestion, and T3 content in tablets was measured by radioimmunoassay. The time to peak serum T3, and the 26 hr intergrated increment in serum T3, Corrected for the amount if T3 ingested, were not significantly different for 75 micrograms of synthetic T3, 6 grains of desiccated thyroid (containing 99 micrograms T3) and 5 grains of thyroglobulin (containing 90 micrograms T3), the mean integrated increment values for the biological preparations being within 12% of those for synthetic T3. The peak serum T4 concentration, the time to peak T4, and 48 hr integrated increments in serum T4 and T3 were similar after 3 mg of Synthroid and Levothroid. The mean peak serum Free T3 Index after 75 micrograms T3, 500, was much higher than the mean peak Free T3 Index after 3 mg T4, 290. The time to peak Free T3 Index was much less after 75 micrograms T3, 2 hr, than the time to peak after 3 mg T4, 2 days. These results indicate that the time course and extent of T3 absorption do not differ, whether the T3 is given as the synthetic iodothyronine or as part of the thyroid protein, thyroglobulin. This approach appears to be useful in determining bioavailability of thyroid hormones from oral preparations and to assess the possibility of thyroid hormone malabsorption.