Clinical, metabolic, and organ-specific indices of thyroid function

Thyroid hormone and the stunned myocardium

Acute critically ill patients experience a rapid decline in plasma free thyroid hormone levels (free triiodothyronine (FT3) and free levothyroxine (FT4)), with a marked elevation of reverse T3, recognized as the euthyroid sick syndrome (ESS) or low-T3 syndrome. The ESS is also often associated with depressed myocardial function, sometimes referred to as the 'stunned myocardium'. Its clinical effects may vary from minimal hemodynamic impairment to cardiogenic shock. Medical management may range from aspirin alone to placement of a left ventricular assist device. With adequate supportive therapy, recovery usually occurs within days or weeks. The effect of T3/T4 therapy has been studied in three conditions in which the ESS and myocardial functional depression have been documented - i) transient regional myocardial ischemia and reperfusion, ii) transient global myocardial ischemia in patients undergoing cardiac surgery on cardiopulmonary bypass, and iii) transient inadequate global myocardial perfusion in brain-dead potential organ donors. Under all three conditions, myocardial ischemia leads to rapid loss of high-energy phosphates, accumulation of myocardial tissue lactate, and probably loss of homeostasis of cytosolic calcium, which may further increase cell injury. There is an inability to generate ATP through the Krebs cycle, which reduces the high-energy phosphate pool essential for all cell ATPases. Under all three conditions, following administration of T3/T4, the myocardial dysfunction was rapidly reversed. We, therefore, cautiously advocate the use of thyroid hormonal therapy to any patient with the ESS and/or a stunned myocardium.

The effects of experimentally induced hyperthyroidism on the diving physiology of harbor seals (Phoca vitulina)

Many phocid seals are expert divers that remain submerged longer than expected based on estimates of oxygen storage and utilization. This discrepancy is most likely due to an overestimation of diving metabolic rate. During diving, a selective redistribution of blood flow occurs, which may result in reduced metabolism in the hypoperfused tissues and a possible decline in whole-body metabolism to below the resting level (hypometabolism). Thyroid hormones are crucial in regulation of energy metabolism in vertebrates and therefore their control might be an important part of achieving a hypometabolic state during diving. To investigate the effect of thyroid hormones on diving physiology of phocid seals, we measured oxygen consumption, heart rate, and post-dive lactate concentrations in five harbor seals (Phoca vitulina) conducting 5 min dives on command, in both euthyroid and experimentally induced hyperthyroid states. Oxygen consumption during diving was significantly reduced (by 25%) in both euthyroid and hyperthyroid states, confirming that metabolic rate during diving falls below resting levels. Hyperthyroidism increased oxygen consumption (by 7–8%) when resting in water and during diving, compared with the euthyroid state, illustrating the marked effect of thyroid hormones on metabolic rate. Consequently, post-dive lactate concentrations were significantly increased in the hyperthyroid state, suggesting that the greater oxygen consumption rates forced seals to make increased use of anaerobic metabolic pathways. During diving, hyperthyroid seals also exhibited a more profound decline in heart rate than seals in the euthyroid state, indicating that these seals were pushed toward their aerobic limit and required a more pronounced cardiovascular response. Our results demonstrate the powerful role of thyroid hormones in metabolic regulation and support the hypothesis that thyroid hormones play a role in modulating the at-sea metabolism of phocid seals.

Monitoring endocrine function

This article reviews current knowledge concerning the monitoring of endocrine function in patients in the clinical setting. Monitoring techniques are discussed and literature is reviewed regarding diabetes mellitus, thyroid, and parathyroid disorders, pheochromocytoma, adrenal insufficiency, and carcinoid tumors.

Hashimoto's thyroiditis in children and adolescents

INTRODUCTION Hashimoto's thyroiditis (HT) is a common cause of goitre and hypothyroidism in children and adolescents. Spontaneous remission may occur in up to 50% patients, but the development of hypothyroidism is possible. OBJECTIVE We investigated the clinical manifestations, course and long-term outcome of HT. METHOD We reviewed charts of 43 children (36 females) with HT, mean age at presentation 12.3 years, and mean follow-up duration 4.6 years. RESULTS HT is five times more common in females. The common complaints leading to referral were goitre in 19 children (44.3%), diffuse in 17 children (89.5%). As to the prevalence of goitre, it accounted for significantly more referrals in females (14 girls, and 5 boys; 73.7% vs 26.3%, t-test; p<0.005). Goitre was either isolated in 15 (34.4%) or associated with other complaints in 4 children: anemia in 7 (16.2%), fatigue in 5 (11.8%), increased appetite in 4 (9.7%), weight gain in 3 (7.0%), growth retardation in 2 children (4.7%), at irregular menses in 3 pubertal girls. Hypothyroidism was present in 18 patients (41.1%), 7 (38.8%) on initial admission, and 11 (61.8%) had the mean follow-up duration of 4.6 years. There were 25 euthyroid HT patients (59.9%). The family history of the thyroid disease was positive in 16 children (37.1%) and 12 of them (71.4%) had hypothyroidism. There were 6 patients (13.9%) in whom the disease was associated with some other autoimmune disease. CONCLUSION HT is five times more common in females. The usual complaints leading to referral were diffuse goitre, which accounted for significantly more referrals in females. A positive family history of autoimmune thyroid disease is associated with a higher risk of hypothyroidism in children with HT. Hypothyroid patients may appear in higher percentage of children and adolescents than previously reported.

Thyroid hormone and blood pressure regulation

Thyroid hormone has well-recognized effects on the cardiovascular system and blood pressure regulation. Blood pressure is altered across the entire spectrum of thyroid disease. The effects of hyperthyroidism include increased cardiac output, contractility, tachycardia, widened pulse pressure, decreased systemic vascular resistance, and increased basal metabolic rate. The manifestations of hypothyroidism are in marked contrast to those of hyperthyroidism and include decreased cardiac output, narrow pulse pressure, increased systemic vascular resistance, and decreased metabolic rate. Although thyroid hormone affects almost all tissues of the body and mediates changes in homeostasis, adaptations of the cardiovascular system can result in changes in blood pressure to accommodate the new demands on the system. In this paper, we review the direct and indirect thyroid hormone-mediated effects on blood pressure.

Evaluation of the therapeutic efficacy of different levothyroxine preparations in the treatment of human thyroid disease

At the present time, optimal therapy for hypothyroidism requires replacement of the deficiency in thyroid hormone with synthetic levothyroxine. Precise titration of this narrow therapeutic index drug is necessary to return the patient to a chemically and clinically euthyroid state. Seven levothyroxine formulations are Food and Drug Administration (FDA)-approved and four are available to the physician. Proper dosage is established based on thyrotropin (TSH) testing and clinical evaluation. Each levothyroxine preparation must comply with FDA standards for bioavailability but may vary with respect to its dissolution and absorption properties and are not interchangeable. This equivalence testing is done on normal volunteers and requires a suprapharmacologic dose of levothyroxine in order to make the determination of bioavailability. In this review we discuss the various methods to evaluate therapeutic efficacy and bioequivalence of levothyroxine preparations in the treatment of thyroid disease. These are relevant to the physician and patient because small differences in the efficacy can produce unwanted effects of either underreplacement or overreplacement.