Systolic time intervals and ankle reflex time in patients with minimal serum TSH elevation: response to triiodothyronine therapy

A guide to interrogating immunometabolism

The metabolic charts memorized in early biochemistry courses, and then later forgotten, have come back to haunt many immunologists with new recognition of the importance of these pathways. Metabolites and the activity of metabolic pathways drive energy production, macromolecule synthesis, intracellular signalling, post-translational modifications and cell survival. Immunologists who identify a metabolic phenotype in their system are often left wondering where to begin and what does it mean? Here, we provide a framework for navigating and selecting the appropriate biochemical techniques to explore immunometabolism. We offer recommendations for initial approaches to develop and test metabolic hypotheses and how to avoid common mistakes. We then discuss how to take things to the next level with metabolomic approaches, such as isotope tracing and genetic approaches. By proposing strategies and evaluating the strengths and weaknesses of different methodologies, we aim to provide insight, note important considerations and discuss ways to avoid common misconceptions. Furthermore, we highlight recent studies demonstrating the power of these metabolic approaches to uncover the role of metabolism in immunology. By following the framework in this Review, neophytes and seasoned investigators alike can venture into the emerging realm of cellular metabolism and immunity with confidence and rigour.

Thyroid hormone replacement for central hypothyroidism: a randomized controlled trial comparing two doses of thyroxine (T4) with a combination of T4 and triiodothyronine

BACKGROUND

Dosage of T(4) in central hypothyroidism is primarily guided by the free serum T(4) level (fT4). However, the optimum fT4 range is ill defined, and subtle hypothyroidism might be missed using this approach.

OBJECTIVES

Our aim was to investigate the effects of a body weight (bw)-adapted T(4) treatment, alone or in combination with T(3), on metabolism, well-being, and cognitive function in comparison to a regimen leading to normal fT4.

DESIGN

This was a placebo-controlled trial (double-blind, crossover).

PATIENTS

A total of 29 patients (age 52 +/- 2 yr; females/males, 8/21) with hypopituitarism, including TSH deficiency, participated in the study.

INTERVENTIONS

Three regimens were compared (5 wk each): "EMPIRICAL-T4," empirical T(4) dosage (1 +/- 0.05 microg/kg bw) leading to normal fT4; BW-ADAPTED-T4 (1.6 microg/kg bw T(4)); and "BW-ADAPTED-T3T4," bw-adapted combination of T(3) and T(4) (ratio of 1:10).

RESULTS

BW-ADAPTED-T4 administration increased mean fT4 concentrations to the upper limit of the normal range (peak levels). Compared with EMPIRICAL-T4, BW-ADAPTED-T4 treatment resulted in a lower body mass index (BMI) (29.0 +/- 0.7 vs. 29.5 +/- 0.7 kg/m(2); P < 0.03), lower total cholesterol (198 +/- 9 vs. 226 +/- 7 mg/dl; P < 0.01), and lower low-density lipoprotein (LDL) cholesterol (116 +/- 5 vs. 135 +/- 7 mg/dl; P < 0.01). BW-ADAPTED-T3T4 treatment was associated with additional beneficial effects on ankle reflex time and working memory but resulted in supraphysiological free serum T(3) (fT(3)) levels.

LIMITATIONS

Long-term side effects may have been missed.

CONCLUSIONS

Using a dose of 1.6 microg/kg bw improved markers commonly associated with central hypothyroidism. This suggests that T(4) dosage based on bw and aiming at fT4 in the upper reference range is superior to titration of T(4) aiming at middle normal fT4 concentrations in those patients.

Does Treating Subclinical Hypothyroidism Improve Markers of Cardiovascular Risk?

Subclinical hypothyroidism is defined as an elevated serum thyroid-stimulating hormone (TSH) level in the face of normal free thyroid hormone values. The overall prevalence of subclinical hypothyroidism is 4-10% in the general population and up to 20% in women aged >60 years. The potential benefits and risks of therapy for subclinical hypothyroidism have been debated for 2 decades, and a consensus is still lacking. Besides avoiding the progression to overt hypothyroidism, the decision to treat patients with subclinical hypothyroidism relies mainly on the risk of metabolic and cardiovascular alterations. Subclinical hypothyroidism causes changes in cardiovascular function similar to, but less marked than, those occurring in patients with overt hypothyroidism. Diastolic dysfunction both at rest and upon effort is the most consistent cardiac abnormality in patients with subclinical hypothyroidism, and also in those with slightly elevated TSH levels (>6 mIU/L). Moreover, mild thyroid failure may increase diastolic blood pressure as a result of increased systemic vascular resistance. Restoration of euthyroidism by levothyroxine replacement is generally able to improve all these abnormalities. Early clinical and autopsy studies had suggested an association between subclinical hypothyroidism and coronary heart disease, which has been subsequently confirmed by some, but not all, large cross-sectional and prospective studies. Altered coagulation parameters, elevated lipoprotein (a) levels, and low-grade chronic inflammation are regarded to coalesce with the hypercholesterolemia of untreated patients with subclinical hypothyroidism to enhance the ischemic cardiovascular risk. Although a consensus is still lacking, the strongest evidence for a beneficial effect of levothyroxine replacement on markers of cardiovascular risk is the substantial demonstration that restoration of euthyroidism can lower both total and low-density lipoprotein-cholesterol levels in most patients with subclinical hypothyroidism. However, the actual effectiveness of thyroid hormone substitution in reducing the risk of cardiovascular events remains to be elucidated. In conclusion, the multiplicity and the possible reversibility of subclinical hypothyroidism-associated cardiovascular abnormalities suggest that the decision to treat a patient should depend on the presence of risk factors, rather than on a TSH threshold. On the other hand, levothyroxine replacement therapy can always be discontinued if there is no apparent benefit. Levothyroxine replacement therapy is usually safe providing that excessive administration is avoided by monitoring serum TSH levels. However, the possibility that restoring euthyroidism may be harmful in the oldest of the elderly population of hypothyroid patients has been recently raised, and should be taken into account in making the decision to treat patients with subclinical hypothyroidism who are aged >85 years.

Assessment of left ventricular diastolic function by radionuclide ventriculography at rest and exercise in subclinical hypothyroidism, and its response to L-thyroxine therapy

Hypothyroidism is associated with intrinsic myocardial changes reflected by alterations in contractility and relaxation. Diastolic function, however, rather than systolic cardiac function, seems to be mostly impaired by thyroid hormone deprivation. Our aim was to evaluate diastolic function at rest and during maximal exercise by means of radionuclide ventriculography in subclinical hypothyroidism before and after restoration of euthyroidism. Ten subclinical hypothyroid patients (50 +/- 8.7 years) (thyroid-stimulating hormone 11 +/- 4.2 microUI/ml) without cardiac disease were studied before and 6 months after levothyroxine (L-T(4)) replacement (thyroid-stimulating hormone 1.9 +/- 1.1 microUI/ml). We compared the basal and post-therapy cardiac parameters with a control group of 14 euthyroid patients (52.5 +/- 10 years) (thyroid-stimulating hormone 2.5 +/- 1.2 microUI/ml). Multigated equilibrium radionuclide ventriculography was performed to assess systolic and diastolic ventricular function. Student's t and paired Student's t tests were applied for statistical analysis. We found a significant difference between the time to peak filling rate (TPFR) at rest before (0.241 +/- 0.002 ms) and after (0.190 +/- 0.012 ms) treatment with L-T(4). A significant difference that disappeared after restoration of euthyroidism was also observed between the basal TPFR values of the subclinical hypothyroid patients and the control group (0.189 +/- 0.01 ms). The same pattern was observed at maximal exercise. Thus, TPFR, a parameter of left ventricular (LV) diastolic function measured by radionuclide ventriculography, is impaired in subclinical hypothyroid patients both at rest and during exercise and returns to normal values after L-T(4) therapy.

Hemodynamic changes after levothyroxine treatment in subclinical hypothyroidism

In hypothyroidism, lack of thyroid hormones results in reduced cardiac function (cardiac output [CO]), and an increase of systemic vascular resistance (SVR). We speculated whether hemodynamic regulation in subjects with subclinical hypothyroidism (SH) (defined as mildly elevated thyrotropin [TSH] despite free thyroxine [T(4)] and triiodothyronine [T(3)] estimates within reference range) would benefit from levothyroxine (LT(4)) substitution. CO was measured by impedance cardiography, which is an observer independent method with high precision, and mean arterial pressure (MAP) by oscillometry. SVR was then calculated as MAP/CO. Measurements were performed before and after 60 degrees head-up tilting, and before and after LT(4) substitution. Plasma levels of catecholamines were also measured. In 16 otherwise healthy women with SH (ages 44-74 years; serum TSH in mean 17.1 mU/L (range, 6.8-27), and normal free T(4) and T(3) estimates) LT(4) treatment resulted in 6% reduction in supine MAP (p < 0.01), 14% increase in upright CO (p < 0.05), and 13%-20% decrease in SVR (supine and upright position, respectively) (p < 0.05). Plasma norepinephrine as well as epinephrine decreased during LT(4) treatment (p < 0.05). These changes were qualitatively similar but quantitatively less pronounced than in 15 women with overt hypothyroidism, also studied. Taking the two groups together (n = 31), pretreatment thyroid function (expressed as either TSH or free T(4) estimate) correlated to CO and SVR as well as the changes induced by LT(4) (p < 0.05), i.e., the lower the thyroid function the lower the CO and the higher the SVR, and the greater the response to LT(4). We conclude, that LT(4) treatment in SH results in changes in hemodynamic parameters of potentially beneficial character. SH and overt hypothyroidism should be regarded as a continuum, and our data favor earlier and more aggressive treatment of SH.

Management practices among primary care physicians and thyroid specialists in the care of hypothyroid patients

Prospective studies are not available to address various issues commonly encountered in the management of hypothyroid patients. We have conducted a case-based mail survey of American Thyroid Association (ATA) members and primary care providers (PCP) regarding hypothyroidism management issues. A majority of ATA members and a minority of PCPs used antithyroid antibody testing in the evaluation of hypothyroidism. Approximately 2/3 of all respondents indicated that they would treat patients with mild thyroid failure when antithyroid antibodies are negative; 77% of PCPs and 95% of ATA members recommended treatment when antibodies are positive. For a young patient with mild thyroid failure, 71% of ATA members would initiate a full levothyroxine (LT4) replacement dose of 1.6 microg/kg per day or slightly lower; PCPs were more likely to start with a low dose and titrate upwards. For a young patient with overt hypothyroidism, 42% of PCPs and 51% of ATA respondents recommended an initial full LT4 replacement dose. The majority of all respondents would start with a low LT4 dose and adjust the dose gradually in an elderly patient, regardless of the severity of thyroid hormone deficiency. More than 40% of ATA respondents chose a target thyrotropin (TSH) range of 0.5-2.0 microU/mL for a young patient while 39% favored a goal of 1.0-4.0 microU/mL for an elderly patient. PCPs more often chose a broader TSH goal of 0.5-5.0 microU/mL. In conclusion, the current practice patterns of PCPs and ATA members that were elicited in this survey differ significantly in regard to the evaluation and management of hypothyroidism.

Cardiovascular and atherogenic aspects of subclinical hypothyroidism

Subclinical hypothyroidism (SH) is common, especially among elderly women. There is no clear evidence to date that SH causes clinical heart disease. However, mild thyroid gland failure, evidenced solely by elevation of the serum thyrotropin (TSH) concentration, may be associated with increased morbidity, particularly for cardiovascular disease, and subtly decreased myocardial contractility. In SH, both cardiac structures and function remain normal at rest, but impaired ventricular function as well as cardiovascular and respiratory adaptation to effort may become unmasked during exercise. These changes are reversible when euthyroidism is restored. Flow-mediated vasodilatation, a marker of endothelial function, is significantly impaired in SH, and decreased heart rate variability, a marker of autonomic activity, suggests hypofunctional abnormalities in the parasympathetic nervous system. SH does result in a small increase in low-density lipoprotein (LDL) cholesterol (C) and a decrease in high-density lipoprotein (HDL)-C, changes that enhance the risk for development of atherosclerosis and coronary artery disease (CAD). After coronary revascularization, a trend toward higher rates of chest pain, dissection, and reocclusion has been noted in SH subjects. Smoking may contribute to the high incidence of SH and may aggravate its metabolic effects. Subjects with SH with marked TSH elevation and high titers of thyroid autoantibodies are at higher risk of unnoticed progression to overt hypothyroidism. Especially women over 50 years with TSH levels greater than 10 mU/L and smoking habits have the highest risk for cardiovascular complications. The magnitude of the lipid changes and the subtle impairment of left ventricular function and cardiopulmonary exercise capacity in SH may justify use of hormone replacement. Early levothyroxine (LT4) treatment in SH may reduce the C level by an average of 8% and normalize all metabolic effects in smokers, nevertheless, in some patients, LT4 therapy may exacerbate angina pectoris or an underlying cardiac arrhythmia. Longitudinal follow-up to define the actual cardiovascular disease risk associated with SH is warranted.

Left ventricular diastolic dysfunction in patients with subclinical hypothyroidism

Although subclinical hypothyroidism is frequently diagnosed, the decision to institute a substitutive therapy with L-T4 remains controversial. Because the cardiovascular system is considered a main target for the action of thyroid hormone, we investigated whether subclinical hypothyroidism induces cardiovascular abnormalities. Twenty-six patients (mean age, 36 +/- 12 yr) were evaluated by Doppler-echocardiography, whereas a subgroup of 10 patients, randomly selected, were reevaluated after 6 months of L-T4 substitutive therapy (mean dose, 68 microg daily). Thirty subjects (matched for age, sex, and body surface area) served as controls. Mean plasma TSH was significantly higher in patients (P < 0.001), whereas mean serum free T4 and free T3 concentrations, although in the normal range, were significantly lower (P < 0.001 and P < 0.005, respectively). Blood pressure and heart rate did not differ from control values. Echocardiogram examination showed no abnormalities of the left ventricular morphology and a slight, but not significant, reduction in the systolic function in the patient group. In contrast, Doppler-derived indices of diastolic function showed significant prolongation of the isovolumic relaxation time (94 +/- 13 vs. 84 +/- 8 msec; P < 0.001), increased A wave (55 +/- 13 vs. 48 +/- 9 cm/sec; P < 0.05), and reduced early diastolic mitral flow velocity/late diastolic mitral flow velocity ratio (1.4 +/- 0.3 vs. 1.7 +/- 0.3; P < 0.001). In the subgroup of 10 patients, thyroid hormone profile was normalized by 6 months of L-T4 substitutive therapy, whereas no changes were observed in the left ventricular morphology. Systolic function was significantly enhanced, as compared with pretreatment values (P < 0.01) but did not differ from control values. Also, systemic vascular resistance was significantly decreased by L-T4 replacement therapy. Assessment of diastolic function showed significant shortening of isovolumic relaxation time (77 +/- 15 vs. 91 +/- 8; P < 0.05), reduction of A wave (51 +/- 13 vs. 60 +/- 12; P < 0.01), and increase of early diastolic mitral flow velocity/late diastolic mitral flow velocity ratio (1.7 +/- 0.4 vs. 1.3 +/- 0.3; P < 0.001). These indices, however, were comparable with those of control subjects. These findings indicate that subclinical hypothyroidism affects diastolic function and that this abnormality may be reversed by L-T4 substitutive therapy.

Stapedial reflex: a biological index found to be abnormal in clinical and subclinical hypothyroidism

There is only limited evidence for biological parameter abnormalities in subclinical hypothyroidism. The aim of this study was to investigate the impact of varying degrees of thyroid failure on the stapedial reflex as a biological index, and establish its role in the evaluation of the hypothyroid patient. We studied 10 patients with subclinical hypothyroidism, 10 patients with clinical hypothyroidism, and 20 controls. All three parameters of stapedial reflex (amplitude, decay, and threshold) were measured before and after restoration of euthyroidism through thyroxine administration. Data are given as mean +/- SEM. Stapedial reflex maximal amplitudes were different among the groups studied (p < 0.0001), as values in subclinical (4.3 +/- 0.4 mm) and clinical (3.7 +/- 0.3 mm) groups before treatment were lower (p < 0.05) than those of control (5.7 +/- 0.3 mm), and subclinical (6.4 +/- 0.5 mm) and clinical (5.6 +/- 0.4 mm) groups after treatment. Similarly, stapedial reflex decays were different among the groups studied (p < 0.001), as values in subclinical (81 +/- 7 ms) and clinical (89 +/- 4 ms) groups before treatment were higher (p < 0.05) than those in control (65 +/- 2 ms), subclinical (56 +/- 8 ms), and clinical (61 +/- 8 mm) groups after treatment. There was no significant difference among the groups for stapedial reflex threshold or significant correlation between stapedial reflex parameters and thyroid function tests. Stapedial reflex, a biological parameter that reflects neuromuscular status, is abnormal in patients with subclinical and clinical hypothyroidism and returns to normal when clinical and biochemical euthyroidism has been achieved through thyroxine administration.

Regulation of thyroid hormone receptor-mediated transcription by a cytosol protein

Thyroid hormone receptors (TRs) are members of the steroid hormone/retinoic acid receptor superfamily, which regulate homeostasis, development, and differentiation. Their transcriptional activity is modulated by the thyroid hormone 3,3',5-triiodo-L-thyronine (T3). The present study evaluated the effect of the availability of cytoplasmic T3 on the modulation of transcriptional responses of the TRs. In human choriocarcinoma JEG-3 and monkey COS-1 cells, the cytosolic thyroid hormone binding protein is a monomer of the tetrameric pyruvate kinase, subtype M2, which does not bind T3. The in vivo monomer-tetramer interconversion is regulated by glucose via fructose 1,6-bisphosphate. At the physiological T3 concentration, lowering the glucose concentration led to an increase in the cellular concentration of the cytosolic thyroid hormone binding protein. By using a transient transfection system, a concomitant reduction in the transcriptional activity of the human beta 1 thyroid hormone receptor was detected in both cell lines. In the absence of glucose, the transcriptional activity of the human beta 1 thyroid hormone receptor in JEG-3 and COS-1 cells was reduced by 65-75% and 90-95%, respectively. However, glucose had no effect on the basal transcriptional activity. These findings demonstrate an important prenuclear step in the modulation of the gene regulating activity of the TRs.

Spectrum of subclinical and overt hypothyroidism: effect on thyrotropin, prolactin, and thyroid reserve, and metabolic impact on peripheral target tissues

PURPOSE

Subclinical hypothyroidism is found in about 7.5% of females and in about 3% of males. It appears to be a risk factor for atherosclerosis and for coronary heart disease and can affect various other target organs. The morbidity and clinical significance of subclinical hypothyroidism are controversial. Therefore, we evaluated the metabolic impact of progressive thyroid failure in patients with various degrees of hypothyroidism compared with control subjects.

PATIENTS AND METHODS

We investigated 86 female patients with the whole spectrum of subclinical hypothyroidism (n = 69) and of overt hypothyroidism (n = 17) and 52 euthyroid women as controls. All subjects underwent full medical and endocrine evaluations (including measurements of thyrotropin [TSH], TSH beta- and alpha-subunits, and prolactin before and after oral administration of thyrotropin-releasing hormone [TRH]) as well as lipid profiles and different tests of peripheral thyroid hormone action. All hypothyroid patients were divided into five categories according to disease severity: grades I to III (subclinical hypothyroidism, with normal thyroxine [T4] levels) and grades IV and V (overt hypothyroidism, with diminished T4).

RESULTS

In grade I subclinical hypothyroidism (basal TSH below 6 mU/L), we found significant changes in the clinical index (p less than 0.05), apoprotein A-I level (p less than 0.05), and stimulated prolactin level after oral TRH (p less than 0.001). The findings were similar in grade II (TSH 6 to 12 mU/L). Further changes could be demonstrated in grade III (TSH above 12 mU/L) with a definite elevation of ankle reflex time (p less than 0.001), serum myoglobin level (p less than 0.01), and, to a lesser extent, creatine kinase (p greater than 0.1). The mean low-density lipoprotein cholesterol (LDL-C) level showed an increase of 18%, which was not significant because of marked individual variations (p = 0.15). The frequency of elevated LDL-C levels was definitely higher in patients with grade III disease compared with the controls (42.9% versus 11.4%, p less than 0.05) and with patients with grades I and II disease. Total cholesterol, triglycerides, apoprotein B, and the systolic time intervals (pre-ejection period, corrected for heart rate [PEPc]) were clearly elevated only in overt hypothyroidism (grades IV and V) (p less than 0.01).

CONCLUSION

Subclinical hypothyroidism has significant effects on some peripheral target organs at an early stage (grades I and II), but affects LDL-C, skeletal muscle, and myocardial contractility only at a later stage (grades III, IV, and V). Our data of elevated LDL-C in grade III subclinical hypothyroidism provide a likely pathophysiologic explanation for the reported association of coronary heart disease with this syndrome. The impact of increased prolactin secretion, observed in subclinical hypothyroidism, on gonadal function and infertility has yet to be clarified. Therapy with thyroxine should be recommended in at least some patients with subclinical hypothyroidism. Patients with high TSH levels (above 12 mU/L) will require treatment because of the metabolic effects on several target organs. Before treatment is advocated in all patients with subclinical hypothyroidism, the benefits and long-term side effects of thyroid hormone therapy should be clarified by prospective studies in larger groups of patients.

Anti-iodothyronine autoantibodies in a girl with hyperthyroidism due to pituitary resistance to thyroid hormones

In the present study, we report the uncommon case of a 9.6-yr-old girl with circulating anti-T3 autoantibodies (T3-Ab) and hyperthyroidism due to inappropriate secretion of TSH (IST). The diagnosis of IST was based on the findings of normal TSH levels (2.4 mU/L) in the presence of high free T4 (28.2 pmol/L) and free T3 (FT3) levels, as measured by direct measurement methods based on "one-step" analog tracer (28.0 pmol/L) and "two-step" Lisophase (13.3 pmol/L) techniques. The discrepancy between the two measurements suggested a methodological interference due to T3-Ab in "one-step" technique, being the "two-step" methodology unaffected by the presence of such autoantibodies. T3-Ab were documented by high nonspecific binding of serum to labeled T3 (38.0% vs 4.3 +/- 2.1% in controls). The clinical picture of hyperthyroidism, the qualitatively normal TSH responses to TRH and T3 suppression tests, the normal pituitary imaging and the values of some parameters of peripheral thyroid hormone action compatible with hyperthyroidism indicated that the patient was affected by pituitary resistance to thyroid hormones (PRTH). Chronic treatment with dopaminergic agent bromocriptine (7.5 mg/day) did not cause TSH secretion to be suppressed, while the administration of thyroid hormone analog TRIAC (1.4 mg/day) inhibited TSH release (from 2.4 to 0.2 mU/L). As a consequence, circulating thyroid hormone levels normalized and euthyroidism was restored. During TRIAC administration, FT3 levels, measured by "one-step" analog tracer technique, gave spuriously high values due to the methodological interference of T3-Ab (15.2 vs 4.3 pmol/L as measured by "two-step" Lisophase technique).(ABSTRACT TRUNCATED AT 250 WORDS)

A double-blind cross-over 12-month study of L-thyroxine treatment of women with 'subclinical' hypothyroidism

Twenty women, who had been randomly selected from women with subclinical hypothyroidism identified in a population study were treated with L-thyroxine and placebo in a double-blind cross-over design during 2 x 6 months. Three women did not complete the study, one because she moved to another part of the country, and two because of nervousness and sense of tachycardia. None of these 'drop-outs' had any objective signs of overtreatment; they had normal pulse rate and a serum T3 concentration within the reference interval. During L-thyroxine treatment serum procollagen-III-peptide concentration increased in 13 women out of the 17 women completing the study and at the end of treatment the mean concentration was significantly raised (P less than 0.001). Serum concentrations of procollagen-III-peptide then correlated with those of free thyroxine (P less than 0.01), total thyroxine (P less than 0.05), and reverse triiodothyronine (P less than 0.05). The same comparison revealed little or no effect on the concentrations of serum creatine kinase activity, transcortin or sex-hormone binding globulin. Heart rate-corrected preejection period and symptom score decreased (P less than 0.05). Four women starting with L-thyroxine showed a marked and prolonged (4-6 months) rise in thyrotrophin concentration during the subsequent placebo period, but remained clinically euthyroid. Four women (of 17) improved during therapy as judged by psychometric testing and their own rating. We could not by pretreatment observations identify these four women apart from serum free and total 3,5,3'-triiodothyronine concentrations in the lower part of the health-associated reference interval. Subclinical hypothyroidism is common among middle-aged and old women, and our findings indicate that approximately one woman in four with this 'subclinical' condition will benefit from L-thyroxine treatment.

End-organ responses to thyroxine therapy in subclinical hypothyroidism

We studied variables known to change with thyroid hormone status in 18 patients with subclinical hypothyroidism before and during treatment with thyroxine in a dose sufficient to restore the plasma TSH response to TRH to normal. There was an associated increase in both plasma total T4 and free T4 within the normal range but plasma total T3 and free T3 were unchanged. As a result of thyroxine treatment there was a small but significant increase (P less than 0.05) in left ventricular ejection fraction (LVEF) with maximal exercise but no significant changes in LVEF at rest and moderate exercise, continuously monitored mean sleeping heart rate, day/night ratios of urinary sodium excretion, peripheral nerve conduction velocities, fasting serum triglycerides, total cholesterol (TC), high density lipoproteins (HDL) or TC/HDL ratios. On this evidence we do not consider that thyroxine replacement therapy is indicated in patients with subclinical hypothyroidism.

Cardiac function in mild and severe primary hypothyroidism

One hundred patients with mild to severe primary hypothyroidism have been analyzed for abnormalities in left ventricular performance utilizing noninvasive techniques. These studies have revealed significant abnormalities in cardiac function which correlated inversely with serum T4 levels. The abnormalities in cardiac function were completely reversible with thyroid hormone therapy. Even in patients with mild subclinical hypothyroidism, significant changes in left ventricular performance were achieved by doses of thyroid hormone which normalized TSH secretion. Therapy with either L-T4 or L-T3 resulted in normal cardiac function though L-T3 produced normal cardiac function on lower doses than were necessary to normalize TSH secretion. These studies are intended to provide new information on the optimal treatment of hypothyroidism.