Relevance of increased serum thyroxine concentrations associated with normal serum triiodothyronine values in hypothyroid patients receiving thyroxine: a case for "tissue thyrotoxicosis"

Serum Thyrotropin and Triiodothyronine Levels in Levothyroxine-treated Patients

CONTEXT

Small adjustments in levothyroxine (LT4) dose do not appear to provide clinical benefit despite changes in thyrotropin (TSH) levels within the reference range. We hypothesize that the accompanying changes in serum total triiodothyronine (T3) levels do not reflect the magnitude of the changes in serum TSH.

OBJECTIVE

This work aims to characterize the relationships of serum free thyroxine (FT4) vs T3, FT4 vs TSH, and FT4 vs the T3/FT4 ratio.

METHODS

This cross-sectional, observational study comprised 9850 participants aged 18 years and older treated with LT4 from a large clinical database from January 1, 2009, to December 31, 2019. Patients had been treated with LT4, subdivided by serum FT4 level. Main outcome measures included model fitting of the relationships between serum FT4 vs TSH, FT4 vs T3, and FT4 vs T3/FT4. Mean and median values of TSH, T3, and T3/FT4 were calculated.

RESULTS

The relationships T3 vs FT4 and TSH vs FT4 were both complex and best represented by distinct, segmented regression models. Increasing FT4 levels were linearly associated with T3 levels until an inflection point at an FT4 level of 0.7 ng/dL, after which a flattening of the slope was observed following a convex quadratic curve. In contrast, increasing FT4 levels were associated with steep declines in TSH following 2 negative sigmoid curves. The FT4 vs T3/FT4 relationship was fit to an asymptotic regression curve supporting less T4 to T3 activation at higher FT4 levels.

CONCLUSION

In LT4-treated patients, the relationships between serum FT4 vs TSH and FT4 vs T3 across a range of FT4 levels are disproportionate. As a result, dose changes in LT4 that robustly modify serum FT4 and TSH values may only minimally affect serum T3 levels and result in no significant clinical benefit.

Individualized Therapy for Hypothyroidism: Is T4 Enough for Everyone?

CONTEXT

It is well recognized that some hypothyroid patients on levothyroxine (LT4) remain symptomatic, but why patients are susceptible to this condition, why symptoms persist, and what is the role of combination therapy with LT4 and liothyronine (LT3), are questions that remain unclear. Here we explore evidence of abnormal thyroid hormone (TH) metabolism in LT4-treated patients, and offer a rationale for why some patients perceive LT4 therapy as a failure.

EVIDENCE ACQUISITION

This review is based on a collection of primary and review literature gathered from a PubMed search of "hypothyroidism," "levothyroxine," "liothyronine," and "desiccated thyroid extract," among other keywords. PubMed searches were supplemented by Google Scholar and the authors' prior knowledge of the subject.

EVIDENCE SYNTHESIS

In most LT4-treated patients, normalization of serum thyrotropin levels results in decreased serum T3/T4 ratio, with relatively lower serum T3 levels; in at least 15% of the cases, serum T3 levels are below normal. These changes can lead to a reduction in TH action, which would explain the slower rate of metabolism and elevated serum cholesterol levels. A small percentage of patients might also experience persistent symptoms of hypothyroidism, with impaired cognition and tiredness. We propose that such patients carry a key clinical factor, for example, specific genetic and/or immunologic makeup, that is well compensated while the thyroid function is normal but might become apparent when compounded with relatively lower serum T3 levels.

CONCLUSIONS

After excluding other explanations, physicians should openly discuss and consider therapy with LT4 and LT3 with those hypothyroid patients who have persistent symptoms or metabolic abnormalities despite normalization of serum thyrotropin level. New clinical trials focused on symptomatic patients, genetic makeup, and comorbidities, with the statistical power to identify differences between monotherapy and combination therapy, are needed.

Transient Osteoporosis of the Hip: Risk and Therapy

Objective

The aim of this study was to investigate the impact of occupation in patients with transient osteoporosis of the hip (TOH). The study also compares two different types of management for this condition: conservative treatment and surgical drilling.

Methods

This was a retrospective case series study. The medical records for patients diagnosed with TOH at our institution within the period 2012–2017 were retrieved. General demographic data, clinical features, and diagnostic modalities were obtained. In addition, management procedures and their associated prognostic factors were acquired. The effectiveness of these procedures was assessed by the number of days of sick leave, the time needed for full recovery and the number of recurrences of TOH. Also, pain responses at 24 hrs, 48 hrs, and at 1 week were estimated subjectively through a “pain score” out of 10, and objectively through the degree of improvement in daily activity. The patients had a regular follow-up at 4- to 6-week intervals.

Results

In total, 15 cases of TOH, 14 men and one woman, were enrolled in the study. The mean age of the patients was 41 years (range 26–59 years). Out of the 15 cases, nine were healthcare professionals (eight physicians and one nurse). Ten patients underwent hip drilling for core decompression and five patients were treated conservatively. The time needed for full recovery was 5.8 weeks for those who underwent drilling, and 48.3 weeks for three patients receiving conservative treatment. The other two patients who were treated conservatively had not achieved full or near-full recovery at the time of reporting this study.

Conclusion

Physicians may be at increased risk of developing TOH. Further studies should be conducted to examine the role of this occupation as a risk factor. In addition, hip drilling should be considered as an effective treatment modality, especially in those patients who seek a faster recovery.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Male transient hip osteoporosis: are physicians at a higher risk?

SUMMARY: Data of 17 male patients with a final diagnosis of transient osteoporosis (TO) of the hip seen at the radiology department of a university hospital over a period of 7 years were retrospectively studied. Comparison of the proportion of doctors among TO cases with that among other hip joint pathologies revealed a significant difference. PURPOSE: The objective of this study is to present clinical and radiological findings of hip TO and to assess the association of the condition with patients' occupation. MATERIALS AND METHODS: Data of 17 male patients with a final diagnosis of transient osteoporosis of the hip seen at the radiology department of a university hospital over a period of 7 years were retrospectively studied. RESULTS: The diagnosis of transient hip osteoporosis was in accordance with the published literature for clinical, imaging findings, and laboratory results. Among the 17 cases of male transient hip osteoporosis, 65% were physicians and 12% were commercial pilots. Comparison of the proportion of doctors among TO cases with that among other hip joint pathologies revealed a significant difference. Abnormal low hormone levels of testosterone and 25-cholicalciferol (vitamin D) were detected in three and two patients, respectively. CONCLUSION: The preponderance of physicians among patients with transient hip osteoporosis has not been previously reported. Etiological contribution of endocrine pathology and type of occupation to the causation of transient osteoporosis merits further research.

The clinical significance of subclinical thyroid dysfunction

Subclinical thyroid disease (SCTD) is defined as serum free T(4) and free T(3) levels within their respective reference ranges in the presence of abnormal serum TSH levels. SCTD is being diagnosed more frequently in clinical practice in young and middle-aged people as well as in the elderly. However, the clinical significance of subclinical thyroid dysfunction is much debated. Subclinical hyper- and hypothyroidism can have repercussions on the cardiovascular system and bone, as well as on other organs and systems. However, the treatment and management of SCTD and population screening are controversial despite the potential risk of progression to overt disease, and there is no consensus on the thyroid hormone and thyrotropin cutoff values at which treatment should be contemplated. Opinions differ regarding tissue effects, symptoms, signs, and cardiovascular risk. Here, we critically review the data on the prevalence and progression of SCTD, its tissue effects, and its prognostic implications. We also examine the mechanisms underlying tissue alterations in SCTD and the effects of replacement therapy on progression and tissue parameters. Lastly, we address the issue of the need to treat slight thyroid hormone deficiency or excess in relation to the patient's age.

Health status, psychological symptoms, mood, and cognition in L-thyroxine-treated hypothyroid subjects

OBJECTIVE

Many hypothyroid subjects receiving L-thyroxine (L-T4) complain of psychological symptoms or cognitive dysfunction. However, there is limited validated information on these self-reports.

DESIGN

Cross-sectional comparison of 20 euthyroid and 34 treated hypothyroid subjects, aged 20-45 years, with normal thyroid-stimulating hormone (TSH) levels. Subjects underwent the following validated measures: Short Form 36 (SF-36); Symptom Checklist 90-Revised (SCL-90-R); Profile of Mood States (POMS); and tests of declarative memory (Paragraph Recall, Complex Figure), working memory (N-Back, Subject Ordered Pointing, Digit Span Backwards), and motor learning (Pursuit Rotor).

MAIN OUTCOMES

L-T4-treated subjects had higher mean TSH and free T4 levels, but free triiodothyronine (T3) levels were comparable to controls. L-T4-treated subjects had decrements on SF-36 and SCL-90-R summary scales and subscales. These subjects performed slightly worse on N-Back and Pursuit Rotor tests. Neither TSH nor thyroid hormone levels were associated with performance on psychological or cognitive measures.

CONCLUSIONS

This group of L-T4-treated subjects had decrements in health status, psychological function, working memory, and motor learning compared to euthyroid controls. Higher mean TSH levels suggest this may be related to suboptimal treatment, although there were no correlations between TSH levels and outcomes. These findings are limited by potential selection bias, and randomized studies targeting different TSH levels and memory subdomains would clarify these issues.

Manifestations et risques cardiovasculaires des dysthyroïdies infracliniques

PURPOSE

To clarify the importance of cardiovascular symptoms and risks in subclinical dysthyroidism in order to define the best way of treatment and follow-up.

CURRENT KNOWLEDGE AND KEY POINTS

Subclinical dysthyroidism is defined by abnormal circulating TSH values in face and normal free thyroid hormones levels, in asymptomatic individuals. If the cardiovascular effects of overt hyperthyroidism are well documented, the relation between subclinical dysthyroidism and the heart is not well established. Subclinical hyperthyroidism may be caused by the same thyroid disorders that results in overt hyperthyroidism, but the most common cause is excessive dosage in levothyroxine. The most frequent cardias complication of subclinical hyperthyroidism is atrial fibrillation. Recently minimal alterations of myocardial function have also been described. In most patients, one tries to return to euthyroidism in order to prevent cardiovascular complications. Subclinical hypothyroidism is 3 to 10 times more frequent, especially in women after 60 years. Subtle modifications of cardiac function and lipid metabolism and an increased risk of atherosclerosis have been described in this condition. There is still debate about the decision to treat or not to treat these patients.

FUTURE PROSPECTS AND PROJECTS

Until now, treatment of subclinical dysthyroidism is mainly based upon experiences and convictions to physicians. Prospective studies are necessary to assess the true benefits and risks of either early treatment or therapeutic abstention with regular clinical and biological follow up. In such studies, patients should be separated according to age and the nature (endogenous or exogenous) of dysthyroidism.

Distinguishing transient osteoporosis of the hip from avascular necrosis

INTRODUCTION

To review the circumstances surrounding the misdiagnosis of transient osteoporosis of the hip (TOH) as avascular necrosis (AVN) and to increase physician awareness of the prevalence and diagnosis of this condition in young men, we reviewed a series of cases seen in the orthopedic unit at St. Michael's Hospital, University of Toronto.

METHODS

We studied the charts of patients with TOH referred between 1998 and 2001 with a diagnosis of AVN for demographic data, risk factors, imaging results and outcomes.

RESULTS

Twelve hips in 10 young men (mean age 41 yr, range from 32-55 yr) were identified. Nine men underwent magnetic resonance imaging (MRI) before referral, which showed characteristic changes of TOH. All 10 patients were referred for surgical intervention for a diagnosis of AVN. The correct diagnosis was made after reviewing patients' charts and the scans and was confirmed by spontaneous resolution of both symptoms and MRI findings an average of 5.5 months and 7.5 months, respectively, after consultation.

CONCLUSIONS

Despite recent publications, the prevalence of TOH among young men is still overlooked and the distinctive MRI appearance still misinterpreted. Symptoms may be severe but resolve over time with reduced weight bearing. The absence of focal changes on MRI is highly suggestive of a transient lesion. A greater level of awareness of this condition is needed to differentiate TOH from AVN, avoiding unnecessary surgery and ensuring appropriate treatment.

Subclinical thyrotoxicosis and the heart

Subclinical thyrotoxicosis is defined as a below normal thyrotropin (TSH) in association with a normal total and free thyroxine (T4) and triiodothyronine (T3). It may be caused by thyroid hormone treatment or by endogenous thyroid disease. The degree of thyrotoxicosis may be estimated by the level of TSH suppression. Subclinical thyrotoxicosis may be associated with changes in cardiac performance and morphology, but this has not been consistently found in all patient populations. Changes may include increased heart rate, increased left ventricular mass index, increased cardiac contractility, diastolic dysfunction, and the induction of ectopic atrial beats or arrhythmias. Cardiac exercise performance may be impaired. Subclinical thyrotoxicosis should be treated in patients with cardiac symptoms or disease. Treatment may include reduction of the thyroid hormone dose, treatment of the underlying thyroid condition, or beta-blockers.

Effects of thyroid hormone on erythrocyte carbonic anhydrase-I and zinc concentrations in vivo and in vitro: clinical usefulness of carbonic anhydrase-I and zinc concentrations in erythrocytes

Red blood cell (RBC) carbonic anhydrase-I isozyme (CAI) and zinc concentrations were measured in patients with various thyroid diseases. Both concentrations were decreased in patients with hyperthyroid Graves' disease, and the RBC CAI concentration significantly (r = 0.95) correlated with the RBC zinc concentration. After treatment, the normalization of RBC CAI and zinc lagged 2 months behind normalization of plasma T4 and T3 levels. Furthermore, the highest correlation coefficients were observed between RBC CAI and zinc levels and plasma thyroid hormone levels measured 8 weeks earlier. These results indicate that both RBC CAI and zinc levels reflect the integrated plasma thyroid hormone levels over the previous few months. Transient thyrotoxicosis due to destructive (subacute or painless) thyroiditis did not cause significant changes in the RBC CAI and zinc concentrations. Therefore, the measurement of RBC CAI and zinc concentrations may be useful in differentiating patients with Graves' disease from those with transient thyrotoxicosis. T3 at a physiological free concentration significantly decreased CAI concentration in erythroleukemic YN-1 cells and burst forming unit-erythroid-derived cells obtained by culturing peripheral blood mononuclear cells. These cells may be useful in vitro models for investigations of thyroid hormone action in human erythroid cells.

Cardiac hypertrophy as a result of long-term thyroxine therapy and thyrotoxicosis

OBJECTIVES

To define the effects of long-term thyroxine treatment upon heart rate, blood pressure, left ventricular systolic function, and left ventricular size, as well as indices of autonomic function, and to compare findings with those in patients with thyrotoxicosis before and during treatment.

DESIGN

Cross sectional study of patients prescribed thyroxine long term (n = 11), patients with thyrotoxicosis studied at presentation (n = 23), compared with controls (n = 25); longitudinal study of patients with thyrotoxicosis studied at presentation and serially after beginning antithyroid drug treatment (n = 23).

METHODS

24 h ambulatory monitoring of pulse and blood pressure, echocardiography, forearm plethysmography, and autonomic function tests.

RESULTS

Long-term thyroxine treatment in doses that reduced serum thyrotrophin to below normal had no effect on blood pressure, heart rate, left ventricular systolic function or stroke volume index, but was associated with an 18.4% increase in left ventricular mass index (mean (SEM) 101.9 (3.09) g/m2 v controls 86.1 (4.61), P < 0.01). Thryoxine treatment, like thyrotoxicosis, had no effect on tests of autonomic function. Untreated thyrotoxicosis resulted in pronounced changes in systolic and diastolic blood pressure and an increase in heart rate during waking and sleep. Patients with thyrotoxicosis at presentation had an increase in left ventricular systolic function (ejection fraction 70.5 (1.66)% v 65.4 (1.79), P < 0.01; fractional shortening 40.4 (1.54)% v 35.6 (1.46), P < 0.01), increased stroke volume index (45.9 (2.4) ml/m2 v 36.6 (1.7), P < 0.001), and an increase in forearm blood flow, and decrease in vascular resistance. They had a similar degree of left ventricular hypertrophy to that associated with thyroxine treatment (99.3 (4.03) g/m2); all changes were corrected within 2 months by antithyroid drugs.

CONCLUSIONS

The development of left ventricular hypertrophy in patients receiving thyroxine in the absence of significant changes in heart rate, blood pressure, and left ventricular systolic function is consistent with a direct trophic effect of thyroid hormone on the myocardium. The presence of left ventricular hypertrophy determines that further studies are essential to assess cardiovascular risk in patients taking thyroxine long term.

Thyroid disease: effects on cardiovascular function

Thyroid hormones exert direct effects upon the heart and indirect influences mediated via changes in cardiac work. Overt thyroid dysfunction is frequently associated with cardiovascular symptoms and signs and less frequently with significant cardiovascular morbidity due to atrial fibrillation or cardiac failure. Whether subclinical thyroid dysfunction is similarly associated with adverse effects upon the cardiovascular system remains unclear.

Subclinical thyroid disease

Thyroid disease can roughly be divided into functional and anatomical disorders. Subclinical disease is by definition not accompanied by symptoms or signs and usually goes unrecognized for the bearer (and the observer). In this communication an overview will be given of existing literature and some own results concerning subclinical hypothyroidism, subclinical thyrotoxicosis and thyroid incidentalomas. Apart from definitions, data on prevalence, clinical effects, prognostic significance and the need for and response to therapy will be discussed.

Outcome of lower L-thyroxine dose for treatment of congenital hypothyroidism

The appropriateness of the recommended L-thyroxine dose (10-15 micrograms/kg/day) for the treatment of congenital hypothyroidism has been questioned because of the risk of iatrogenic hyperthyroidism. We report the outcome of 23 newborns with congenital primary hypothyroidism treated with 25 micrograms L-thyroxine per day (5.3-9.2 micrograms/kg/day) and followed for an average of 59 months. Serum thyroxine (T4) values increased (X = 11.4 +/- 2.7 micrograms/dL) within 4 weeks posttherapy; eight infants had T4 levels > or = 13 micrograms/dL on only half the currently recommended dose. Thyroid-stimulating hormone (TSH) values remained elevated in 18 of 21 patients for 2-21 months despite a high-normal T4. Psychometric tests were performed in 19 of the 23 patients. The mean Full Scale IQ for the congenital hypothyroid group (n = 16) was 101.4 +/- 13.2 with comparable Verbal and Performance IQ scores. Patients with a bone age (BA) of < or = 32 weeks or T4 < 2 micrograms/dL at initial evaluation had significantly Lower Verbal IQ scores. A standardized parent-report assessment of behavioral and emotional functioning revealed subgroup scale scores that were indistinguishable from nonclinical norms. We conclude that (1) average range IQ scores and positive behavioral adaptation are observed in congenitally hypothyroid children treated with L-thyroxine doses lower than currently recommended; (2) the L-thyroxine dose should be individualized to prevent iatrogenic hyperthyroidism; (3) TSH normalization should not be a primary objective of treatment, and (4) a prospective study comparing the advantages and risks of different doses of L-thyroxine is needed.

Free thyroid hormones and a third-generation TSH assay in the detection of hyperthyroidism during long-term thyroxine treatment in thyroid carcinoma patients

We evaluated the value of serum-free thyroid hormone and thyrotropin (TSH) concentrations in the detection of peripheral hyperthyroidism during thyroxine suppression therapy. A total of 57 patients on a stable thyroxine dose and 70 controls participated in the study. Serum-free thyroxine (FT4), free triiodothyronine (FT3) and TSH were measured by immunoassays based on time-resolved fluorescence (Delfia). The assay for TSH was a modification of a third generation Delfia hTSH Ultra method. The patients were classified into euthyroid and hyperthyroid subgroups based on clinical signs and symptoms (Wayne index). Systolic time intervals (STI) were measured. The Wayne indices were higher among patients than controls (p < 0.0001). The STI results were similar in patients and controls. Only FT4 had the discriminatory power for classifying euthyroid and hyperthyroid patients according to discriminant analyses. The diagnostic value of FT4 was further assessed by calculating the area under the relative operating characteristic (ROC) curve. The area was 0.707 (SE 0.0918), which was significantly different from an area of 0.5, i.e. the area of a test of no value (p = 0.032). In conclusion, a high serum FT4 concentration indicates hyperthyroidism during long-term thyroxine treatment among thyroid carcinoma patients. Although the degree of TSH suppression can now be exactly monitored with new third generation TSH assays, hyperthyroidism cannot be defined using TSH concentration in thyroid carcinoma patients. Therefore, additional serum FT4 concentration assays are needed in the assessment of hyperthyroidism associated with TSH suppression therapy in thyroid carcinoma patients.

Bone mineral density and metabolism in premenopausal women taking L-thyroxine replacement therapy

BACKGROUND AND OBJECTIVE

Excess endogenous thyroxine causes bone loss, but the effects of exogenous thyroxine are disputed. We report on bone mass and metabolism in women taking L-thyroxine therapy.

DESIGN

Cross-sectional and longitudinal studies.

PATIENTS

Cross-sectional study: 40 healthy premenopausal women with autoimmune thyroiditis taking either physiological (serum TSH usually normal, 0.35-3.3 mU/l) or suppressive (serum TSH usually < 0.35 mU/l) doses of L-thyroxine; patients were also compared with previously acquired age and weight matched premenopausal volunteers with no history of thyroid dysfunction. Longitudinal study: 28 patients were followed-up > or = 1 year later.

MEASUREMENTS

In all subjects bone mineral density (BMD) was measured at the anteroposterior lumbar spine and left hip (femoral neck, greater trochanter and Ward's area) using dual-energy X-ray absorptiometry, and physical activity assessed using the Framingham physical activity index. Serum osteocalcin (OC), PTH and vitamin D, and urinary pyridinoline and deoxypyridinoline excretion were measured in patients.

RESULTS

Cross-sectional study: The patient groups were well matched for disease duration and physical activity although the suppressed group were slightly younger (mean 38.1 (SD 7.5) vs 43.3 (3.9) years, P < 0.05). BMD, serum OC, PTH and vitamin D, and urinary cross-link excretion did not differ significantly between the two groups. Multivariate analysis of the whole group suggested that BMD at the femoral neck and greater trochanter was related positively to weight and physical activity and negatively to thyroxine dose (microgram/kg/day) and patient age. At the lumbar spine BMD was reduced non-specifically in the presence of thyroid disease and treatment, but this effect appeared to decline with increasing duration of therapy. A similar analysis of the patient group suggested that urinary cross-link excretion correlated positively with thyroxine dose, and negatively with duration of treatment. Longitudinal study: Annualized changes in BMD were inversely related to thyroxine dose (microgram/kg/day) at all sites but achieved statistical significance only at the femoral neck and Ward's area.

CONCLUSION

We did not find any effect of persistent historical TSH suppression on current bone mass, and this might relate to the relative insensitivity of older TSH assays. However, the cross-sectional and longitudinal data suggest that high daily doses of thyroxine in relation to patient body weight might adversely affect bone mass, particularly at the hip. These findings support the contention that excess exogenous thyroxine might predominantly deplete skeletal sites, such as the femoral neck, rich in cortical bone.

TSH as an index of L-Thyroxine replacement and suppression therapy

When hypothalamic-pituitary function is normal, serum TSH levels measured by ultrasensitive assay yield bioassays of endogenous thyroid action and thus provide an ideal index of thyroid secretion and its relationship to fluctuating endogenous thyroid levels. It is theoretically possible that patients receiving exogenous L-thyroxine for primary hypothyroidism should have suppressed TSH levels if physiological needs are constantly met. To examine this possibility free thyroxine, FT4 and TSH were measured in 90 clinically euthyroid patients receiving treatment with L-thyroxine for primary hypothyroidism. TSH levels were normal in 44, suppressed in 16 and elevated in 30 patients. FT4 levels were normal in 68, elevated in 13 and suppressed in 9 patients. Normal TSH levels were associated with normal FT4 levels in 79.5% of patients, elevated FT4 levels in 13.6% and low FT4 in 6.8%. Suppressed TSH levels were associated with elevated FT4 levels in 37.5% of patients and normal FT4 levels in 62.5%. When FT4 levels were normal, however, TSH levels were normal in only 51.5% and abnormal in 48.5%. We also examined the possibility that FT4 levels may remain within normal range when TSH is suppressed during L-thyroxine treatment for goitre or cancer. FT4 and TSH were measured in 45 patients on L-thyroxine as TSH suppression treatment. TSH was suppressed in 23 patients (51.1%), normal in 20 (44.4%) and elevated in 2 (4.5%). When TSH was suppressed, FT4 was elevated in 30.4% but normal in 69.6% of patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac systolic time intervals and thyroid hormone levels during treatment of hypothyroidism

This study was undertaken to compare results of modern serum thyroid hormone assays with cardiac systolic time intervals (STI) during thyroxine treatment in hypothyroid patients. The patients were assessed clinically (Billewicz index) and the STI and serum thyrotropin (TSH), total and free thyroxine (T4) and total and free triiodothyronine (T3) were determined in 16 hypothyroid women (Group I) treated with 50 micrograms increments of thyroxine, and in 13 women who had a history of thyroid carcinoma and high-dose thyroxine replacement therapy and had elevated thyroid hormone concentrations (Group II). The STI of 24 matched healthy female controls were used for reference of STI. The pre-ejection period (PEP) index and the PEP/LVET ratio (left ventricular ejection period) were greater in untreated overtly and mildly hypothyroid patients (p less than 0.05) than in the controls. During stable thyroxine therapy [mean daily dosage for Group I 137.5 (7.3) micrograms and for Group II 220 (61) micrograms] the PEP correlated with serum free T4 (FT4), as measured by a two-step method (SpectriaR) (r = -0.55, p less than 0.01, n = 29) and total T4 (r = -0.51, p less than 0.05, n = 29), but not with TSH, T3, FT3 or FT4 measured by an analogue method Amerlex-M(R). The TRH test was not valuable in follow-up because of the strong correlation between basal TSH and stimulated TSH values (r = 0.95). In conclusion, STI are useful for assessment of the thyroid state in untreated hypothyroid patients. Serum TSH becomes normal in the same time as STI and is the best for follow-up. If serum TSH is low and the patient is on stable thyroxine therapy, we recommend serum FT4 for monitoring thyroxine replacement. Two-step FT4 assays had the best correlation with STI, which has significance in patients with non-thyroidal illness.

Administration of levothyroxine to euthyroid dogs does not affect echocardiographic and electrocardiographic measurements

The effects of a replacement dose of levothyroxine on electrocardiographic and echocardiographic parameters in euthyroid dogs were examined. Two-dimensional guided M-mode echocardiograms and electrocardiograms were obtained before and after administration of levothyroxine (0.5 mg m-2 twice a day) to 10 euthyroid dogs for a period of eight weeks; four untreated dogs acted as controls. The resting serum total thyroxine concentration was significantly increased in treated dogs during weeks 4 to 8. There were no significant differences in any of the echocardiographic or electrocardiographic measurements resulting from treatment.

Evaluation of L-thyroxine replacement therapy in children with congenital hypothyroidism

The outcome of L-thyroxine (L-T4) replacement therapy in children with congenital hypothyroidism (CH) remains to be completely evaluated. In this paper the overall pattern of response to L-T4 replacement therapy was studied in a group of 19 children with CH diagnosed by neonatal screening (10 with hypoplastic/aplastic thyroid disease, group H/A; 9 with gland ectopy, group E) who were followed-up for 60 +/- 27 months (mean +/- SD). With 1 exception serum T4 at diagnosis was greater than 2 micrograms/dl in children of group E and less than 2 micrograms/dl in those of group H/A. The initial dose of L-T4 (8-10 micrograms/kg BW/day) was modified in relation to age and weight in order to maintain serum TSH less than or equal to 5 microU/ml and FT3 in the normal range. A general inverse correlation between serum TSH and FT4 or FT3 concentrations was found, and the mean levels of serum FT4 and FT3 were significantly higher according to the following order of TSH results: low TSH (0-0.5 microU/ml) greater than normal (greater than 0.5-5 microU/ml) greater than elevated TSH (greater than 5 microU/ml). TSH levels less than or equal to 5 microU/ml were associated with FT4 values in the upper half of the normal range (54% of observations) or even higher (46%). Elevation of serum FT4 alone with FT3 values in the normal range did not result in clinical thyrotoxicosis, alteration of growth or premature craniosynostosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of subclinical hyperthyroidism on renal handling of water and electrolytes in patients with nodular goiter

Evidence is beginning to accumulate that minor degrees of hyperthyroidism lead to adverse effects in various tissues, even though clinically the patients are euthyroid. To determine whether these anomalies in thyroid function have deleterious effects on renal function and electrolyte metabolism, the plasma concentrations of electrolytes, urea, and creatinine, the renal handling of water and sodium, and the urinary excretion of these substances were measured in patients with nodular goiter who were displaying stable subclinical hyperthyroidism. The studies were carried out before and after correcting the thyroid dysfunction. Restoration of euthyroidism did not modify any of the renal function parameters studied and did not cause changes in blood analyte levels. The data show that treatment of minor degrees of hyperthyroidism does not have any effects on renal function and electrolyte metabolism, and confirm the well-known capacity of the kidney to adjust its functions to changes induced by an abnormal secretion of thyroid hormones.

Bone mineral density and thyroid hormone therapy

The purpose of this study was to evaluate prospectively the evolution of femoral and vertebral bone mineral density (BMD) in hypothyroid subjects treated with replacement doses (mean +/- SD dose of L-thyroxine = 135 +/- 32 micrograms/day) as compared to an untreated group. Vertebral bone density was also measured in other patients who had been treated for at least 2 years with either suppressive (mean dose = 154 +/- 36 micrograms/day, n = 28) or replacement doses (mean dose = 104 +/- 52 micrograms/day, n = 21) according to the thyrotrophin response (TSH) to the thyrotrophin releasing hormone (TRH) administration. In primary hypothyroid patients, a mean decrease of 5.4% (P less than 0.01) for vertebral BMD, 7.3% (P less than 0.01) for trochanter and 7% (P less than 0.001) for femoral neck was observed after 1 year of treatment. This loss was unrelated either to age or to menopausal status (ANOVA). A clinical and hormonal state of euthyroidism was reached since the 3rd month of treatment. Fasting urinary calcium/creatinine excretion was increased significantly (P less than 0.05) at the 3rd month, and plasma osteocalcin (OC) increased significantly from the 3rd month onwards (P less than 0.05) up to the 12th month (P less than 0.025). In the cross-sectional study, vertebral BMD was not significantly different from age-matched normal values in patients receiving either substitutive or suppressive doses of LT4. These results suggest that in the case of primary hypothyroidism even appropriate thyroid replacement therapy could lead during the first year of treatment to a significant reduction in vertebral and femoral BMD. However, the fact that an increased fracture rate has not been documented in long-term treated patients, and the results of our cross-sectional study, suggest that this bone mass reduction could be transient and reversible due to new bone formation at the end of the resorptive sequence.

Determination of replacement and suppressive doses of thyroxine

Suppression daily doses of thyroxine (T4) were determined and the daily amounts of T4 required to replace T4 were established in 217 hypothyroid patients. Patients with Hashimoto's thyroiditis treated daily with 2-3 micrograms/kg lean body mass or 1-2 micrograms/kg body weight T4 had normal serum thyrotrophin (TSH) concentrations, normal response to TSH-releasing hormone (TRH) and normal systolic time intervals but doses higher than 3 micrograms/kg lean body mass or 2 micrograms/kg body weight decreased serum TSH concentrations, with no response to TRH and systolic time intervals typical of hyperthyroidism. In 13/32 (41%) hypothyroid patients with Graves' disease following 131I and/or surgery, the daily T4 replacement dose was similar to that in Hashimoto's thyroiditis patients but in 12 (38%) patients daily doses of 2-3 micrograms/kg lean body mass or 1-2 micrograms/kg body weight T4 increased serum T4 and suppressed TSH levels, and in six (9%) lower doses were required to control hypothyroidism. The T4 suppression dose for patients with thyroid cancer was more than 3 micrograms/kg lean body mass or 2 micrograms/kg body weight, whereas approximately 30% of non-toxic nodular goitre patients required less than 3 micrograms/kg lean body mass. It is concluded that replacement or suppression doses of T4 should be individually determined and that different criteria should be applied for their calculation depending on the thyroid abnormality.

Hypothyroidism in the elderly

This review describes the changes in thyroid physiology with aging and notes that normal thyroid status is maintained with advanced age. The increased frequency of hypothyroidism in the elderly is stressed with consideration given to the multiple possible etiologies. The natural history of hypothyroidism is considered as evidenced by the various hormonal abnormalities in subjects with autoimmune thyroiditis and patients previously treated with radioiodine. The potential significance of subclinical hypothyroidism is discussed in regard to nonspecific symptoms and subtle cardiovascular manifestations. The problems in the clinical and laboratory diagnosis of hypothyroidism are reviewed with emphasis on the high index of suspicion and cautious interpretation of plasma thyroxine and thyroid-stimulating hormone (TSH) levels required. Finally the potential problems in treating hypothyroidism in the elderly are discussed in regard to dosage requirements and the need to avoid subclinical hypothyroidism by using the ultrasensitive TSH assay.

Evidence for a slow tissue adaptation to circulating thyroxine in patients with chronic L-thyroxine treatment

We measured serum procollagen-III-peptide in 67 women with long-term L-thyroxine treatment and compared the results with age-matched controls. The strong correlation between serum free thyroxine and procollagen-III-peptide concentrations previously found after 6 months of L-thyroxine treatment was not found after long-term treatment. There were slightly higher procollagen-III-peptide concentration values in those chronically treated patients who had high free thyroxine levels but this increase was less marked than in patients previously studied after short-term treatment. An increase in procollagen-III-peptide concentration reflects an increased biosynthesis of collagen III, which is present in connective tissues throughout the body, and our findings may be explained by slow tissue adaptation to increased levels of thyroxine. We conclude that the increased thyroxine levels found in L-thyroxine-treated patients are of less clinical importance than thought previously. We also conclude that peripheral markers of thyroid hormone peripheral effects such as procollagen-III-peptide may be of less use than thought previously due to this slow tissue adaptation to changes in thyroxine concentration.

Low serum TSH with normal total thyroid hormone levels: an indicator of free T4 excess

We have studied 18 patients with low serum thyrotrophin (TSH) levels accompanied by normal thyroxine (T4) and triiodothyronine (T3) levels in order to clarify whether or not they are thyrotoxic. Serum T4, T3, free T3, free T4 and TSH were estimated three times weekly for 3-4 weeks. Thyroxine binding globulin (TBG) levels were estimated on one occasion only. Thyroid hormone data were expressed as the cumulative probability of having at least one result above the relevant normal range by the nth sample (Pn). For free T4 this probability was 61% by the 10th sample. Free T4 values were confined to the upper half of the normal range or above throughout the study. In contrast, free T3 values were distributed evenly within the normal range (P10 = 12%). For total T4 and total T3, P10 was 34 and 11% respectively. Thus, subjects with subnormal TSH levels but normal T4 and T3 levels have been shown to have elevated serum free T4 levels, an indicator of biochemical hyperthyroidism.

Different hepatic responses to thyroxine replacement in spontaneous and 131I-induced primary hypothyroidism

The serum levels of a range of analytes known to change with thyroid status were measured in two groups of patients with primary hypothyroidism commencing T4 replacement therapy. One group (group 1; n = 9) had spontaneous hypothyroidism whilst in the second (group 2; n = 10), hypothyroidism had resulted from radioiodine therapy. The replacement dose was increased in 50 micrograms increments each month to 200 micrograms/day; this produced similar serum concentrations of thyroid hormones and TSH in the two groups at each dose. Dose-dependent increases in glutathione S-transferase (GST) were seen in both groups but changes in alanine aminotransferase (ALT) and gamma glutamyltransferase (GGT) activities occurred only in group 1 patients. Group 1 patients had significantly higher levels of GST than group 2 at the 150 micrograms (P less than 0.01) and the 200 micrograms (P less than 0.005) doses of T4, and they had higher activities of ALT (P less than 0.01) and GGT (P less than 0.02) at the 200 micrograms dose. Seven patients in group 1 had abnormalities in GST and four had high levels of ALT, whereas three patients from group 2 had high GST concentrations and all had ALT activities within reference limits. The concentrations of the other analytes measured in serum showed the same response to T4 in the two groups, particularly the concentrations of certain transport proteins whose serum concentrations depend on hepatic protein synthesis. These data suggest that patients with spontaneous primary hypothyroidism are more susceptible to hepatocellular damage than patients who have radioiodine-induced primary hypothyroidism when given oral doses of thyroxine greater than 150 micrograms/day.

Subclinical hyperthyroidism: possible danger of overzealous thyroxine replacement therapy

Many patients taking customary doses of levothyroxine have slightly elevated serum thyroxine (T4), apparently normal serum triiodothyronine, suppressed serum thyrotropin (thyroid-stimulating hormone; TSH) concentrations, and no clinical symptoms of hyperthyroidism. Recent reports suggest that these patients may have adverse effects from subclinical hyperthyroidism, including abnormally short systolic time intervals, elevations in liver enzymes, and reductions in bone density. Controversy exists about which thyroid function tests should be used to monitor patients taking levothyroxine. A review of currently available data suggests that replacement doses of levothyroxine given to hypothyroid patients should be adjusted so that serum TSH measured by the new sensitive assays is within the normal range. Patients requiring suppressive doses of levothyroxine to shrink goitrous thyroid tissue or to prevent growth of abnormal tissue should be given the minimal dose needed to accomplish the desired clinical or biochemical response.

Use of sensitive immunoradiometric assay for thyrotropin in clinical practice

Although measurement of thyrotropin (thyroid-stimulating hormone; TSH) by radioimmunoassay was a major advance in the laboratory diagnosis of thyroid failure--replacing the time-consuming TSH stimulation test--it was not sufficiently sensitive to discriminate reliably between euthyroid and hyperthyroid patients. Measurement of the TSH response to thyrotropin releasing hormone (TRH) served this purpose, however. The recent development of TSH assays that are severalfold more sensitive and more specific than conventional radioimmunoassays has allowed distinction of euthyroid from hyperthyroid patients and eliminated the need for the TRH test. Although undetectable levels of TSH, compatible with hyperthyroidism, are occasionally noted in euthyroid patients with severe nonthyroidal illness and during the first trimester of pregnancy, false-positive results are less often recorded for TSH than for free or total thyroid hormone measurements. Measurement of TSH by sensitive immunoradiometric assay is currently the most useful first-line test of thyroid function in patients with suspected thyroid disease and, in addition, has a valuable role in monitoring the dose of thyroxine replacement therapy.

Thyroxine uptake by human hepatoma cells from serum of patients submitted to long-term thyroxine suppressive therapy

The significance of thyroxine (T4) uptake from serum in the assessment of thyroid status was evaluated, using human hepatoma (Hep G2) cells, in 30 euthyroid subjects, 6 hypothyroid and 19 hyperthyroid patients, and in 23 athyreotic cancer patients under T4 suppressive therapy. Cellular thyroxine (CT4) was determined according to Sarne and Refetoff, J. Clin. Endocrinol. Metab. 61: 1046, 1985. CT4 averaged 9.9 +/- 2.8 pg/well (mean +/- SD, range 5.7-15.3) in euthyroid subjects, 1.5 +/- 1.0 pg/well (range 0.05-4.2) in hypothyroid patients, 40.5 +/- 18.8 pg/well (range 18.3 +/- 104.7) in hyperthyroid patients, and 23.7 +/- 7.2 pg/well (range 14.2-40.2) in T4-treated patients. In eu-, hypo- and hyperthyroid patients, a significant correlation was observed between CT4 and free T4 index (FT4I), free T4 (FT4) or Sex Hormone Binding Globulin (SHBG) values. In T4-treated patients, CT4 values were correlated with FT4I values, but not with FT4 or SHBG levels. All T4-treated patients with elevated SHBG levels had elevated FT4, FT4I and CT4 values. In contrast, of the 16 T4-treated subjects with normal serum SHBG concentrations, all but one had normal FT3, 3 (19%) had elevated FT4, 10 (62%) elevated FT4I and 13 (81%) elevated CT4, but all (100%) had undetectable TSH levels. Thus, considering serum SHBG concentrations as a parameter of hepatic tissue response to thyroid hormone, CT4 values, at least in T4-treated patients, do not accurately reflect the liver responsiveness to thyroid hormone action.

Restoration of normal thyrotrophin secretion reduces the abnormally high serum glutathione S-transferase levels found in patients receiving thyroxine replacement therapy

The peripheral tissue thyroid status of 12 patients receiving thyroxine replacement therapy was investigated both when pituitary secretion of TSH was suppressed and later, when on a lower dose of thyroxine that restored thyrotroph responsiveness. Heart rate and various analytes in serum known to be sensitive to thyroid status were measured in addition to TSH by immunoradiometric assay. Initially, the serum T4 concentration was raised in seven patients and free T4 raised in nine; all patients had normal T3 concentrations. Later, on the lower dose of thyroxine, most patients had concentrations of thyroid hormones within reference limits. Concentrations of the liver-specific form of glutathione S-transferase (GST) in serum decreased (P less than 0.01) after the reduction in thyroxine dose; abnormally high GST levels, found in eight patients when TSH was suppressed, returned to normal in six of these patients when normal basal and TRH-stimulated TSH concentrations had been restored. The response of the pituitary to excess thyroxine may be more representative of other tissues (e.g. the liver) than previously thought.

The replacement therapy problem in hypothyroidism

There is increasing evidence from studies of heart rate, liver enzyme activity, bone density and urinary sodium excretion that standard replacement therapy doses of thyroxine which suppress TSH secretion are associated with changes in target organ function similar to, but less marked than, those recorded in overt hyperthyroidism. There is also evidence that in subclinical hypothyroidism it is not only the pituitary thyrotroph which recognizes a minor reduction in serum thyroid hormone levels within the normal range. Although there is no proof that slight 'overtreatment' with thyroxine or non-treatment of subclinical hypothyroidism is detrimental to the patient in the long term, the appropriate studies have not been performed. It would seem good clinical practice, however, to treat all grades of thyroid failure and to ensure, if possible, that the dose of thyroxine is adjusted to maintain a normal and detectable TSH level when measured by a sensitive assay system. It must be conceded, however, that with the vagaries of human nature there is always likely to be greater morbidity from patients with hypothyroidism failing to take their medication regularly, than from failure by the medical attendant to make minor adjustments to the dose of thyroxine.

Fine adjustment of thyroxine replacement dosage: comparison of the thyrotrophin releasing hormone test using a sensitive thyrotrophin assay with measurement of free thyroid hormones and clinical assessment

Thyroxine replacement therapy for 21 adult patients with primary hypothyroidism was adjusted to the dosage at which each patient had a normal thyrotrophin (TSH) response to thyrotrophin releasing hormone (TRH). Clinical assessment and measurement of TSH (by sensitive immunoradiometric assay), free thyroxine (FT4) and free tri-iodothyronine (FT3) were made at this dosage and at higher and lower doses of thyroxine. Clinical observations, FT3 and FT4 assays were relatively insensitive to small alterations of thyroxine dosage, in contrast to which basal TSH measurements correlated well with TRH responsiveness and were sensitive to fine adjustments of thyroxine dosage.

Thyroid stimulating hormone measurement by an ultrasensitive assay during thyroxine replacement: comparison with other tests of thyroid function

Serum thyroid stimulating hormone (TSH) was measured using a highly sensitive enzyme-amplified immunoassay in 37 clinically euthyroid patients receiving thyroxine replacement therapy and compared with other biochemical tests of thyroid function. A highly significant correlation (P less than 0.001) was found between the basal serum TSH and the increase in serum TSH concentration 20 min after the administration of thyrotropin releasing hormone (TRH). The basal serum TSH was negatively correlated with the serum total thyroxine (P = 0.05). When patients results were classified as abnormal or normal many discrepancies were noted between the various thyroid tests. A suppressed serum TSH was found in 65% of patients with a normal serum total thyroxine. However, in patients on thyroxine replacement therapy a basal TSH measured by enzyme-amplified immunoassay provides the same information as a TRH test.

Erythrocyte ouabain binding in patients receiving thyroxine

Thyroid hormone action at the cellular level was investigated in euthyroid women who were receiving replacement thyroxine, and whose plasma T4 levels were above the upper reference limit for healthy subjects. There is evidence that erythrocyte sodium pump sites are reduced in number in patients with hyperthyroidism. These sites were measured by the ouabain binding capacity. Plasma T4, free T3 and TSH were also measured, the latter by a high sensitivity fluoroimmunoassay. Three groups of women were investigated; 30 patients receiving T4 with elevated plasma T4 concentrations, 30 age-matched healthy women, and 10 untreated thyrotoxic patients. Erythrocyte ouabain binding was significantly reduced in the thyroxine treated patients, although not to the degree observed in the thyrotoxic patients. Plasma free T3 concentration was increased in 12 of 30 treated patients. TSH was undetectable in 23 of 30 treated patients. The ouabain binding results provide some evidence for increased thyroid hormone action at cellular level in thyroxine treated patients.

Subclinical hyperthyroidism and reduced bone density as a possible result of prolonged suppression of the pituitary-thyroid axis with L-thyroxine

Spontaneous hyperthyroidism and that due to excessive administration of thyroid hormone result in osteopenia. Bone density was measured in 28 white premenopausal female patients who were taking commonly prescribed suppressive doses of L-thyroxine (mean dose 0.171 +/- 0.035 g) for five or more years. The thyroxine level was 13.5 +/- 2.6 micrograms/dl (normal 8.0 +/- 2.4 micrograms/dl), the free thyroxine index was 4.4 +/- 1.0 (normal 2.4 +/- 0.8), and the triiodothyronine value was 154 +/- 26 ng/dl (normal 132 +/- 32 ng/dl). Basal thyrotropin was undetectable (less than 0.08 microIU/ml) in 23 patients, and thyrotropin measured 20 minutes after thyrotropin-releasing hormone administration was not demonstrable in 13 patients and subnormal in 10 patients. Women who had taken L-thyroxine for 10 or more years (n = 12, age 37 +/- 4 years) had a 9 percent reduction in bone density (0.667 +/- 0.044 g/cm2, p less than 0.01) compared with normal premenopausal age-matched control subjects (n = 56, age 35 +/- 6 years, bone density 0.733 +/- 0.055 g/cm2). It is concluded that prolonged suppressive L-thyroxine treatment may result in mild subclinical hyperthyroidism with adverse effects on bone. Patients requiring suppression of the pituitary-thyroid axis should be given the smallest dose of L-thyroxine necessary to achieve a satisfactory clinical response.

Are biochemical tests of thyroid function of any value in monitoring patients receiving thyroxine replacement?

To establish their role in monitoring patients receiving thyroxine replacement biochemical tests of thyroid function were performed in 148 hypothyroid patients studied prospectively. Measurements of serum concentrations of total thyroxine, analogue free thyroxine, total triiodothyronine, analogue free triiodothyronine, and thyroid stimulating hormone, made with a sensitive immunoradiometric assay, did not, except in patients with gross abnormalities, distinguish euthyroid patients from those who were receiving inadequate or excessive replacement. These measurements are therefore of little, if any, value in monitoring patients receiving thyroxine replacement. To stop doing thyroid function tests in these cases would result in considerable savings nationally in the cost of reagents in laboratories using commercial kits.