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Abstract

Thyroid hormones have important thermogenic function. Nevertheless, thyroid dysfunctions are often associated with minor changes in body weight and fat mass. On the other hand, both overfeeding and fasting have important effects on iodothyronine metabolism and regulation of deiodinase activity. Although under debate, there are clinical and theoretical reasons to administer low-dose thyroid hormones (T3) in selected obese patients. This short review deals with both pathophysiological and clinical aspects of thyroid hormone used in the therapy of obesity. International Journal of Obesity (2000) 24, Suppl 2, S116±S119 Keywords: thyroid hormones; obesity; diiodothyronine; UCP Thyroid hormones are prototypes of thermogenetic agentsThe ®rst publication indicating thyroid hormones (TH) as agents involved in energy homeostasis appeared as early as 1895 by Magnus Levy. 1 Although more than 100 y have elapsed since this ®rst publication, the mechanism of calorigenic effect of TH is not fully elucidated and debate is still in progress. Food intakeIt is well established that food intake is generally decreased in hypothyroidism and increased in hyperthyroid state. The reduction in basal metabolic rate (BMR) in hypothyroidism increases the ratio of energy intake to energy expenditure relative to euthyroid subjects consuming the same amount of food. The contrary is true for hyperthyroidism. Body weightBody weight is on average decreased in hyperthyroidism by 15% (in comparison with body weight before) and an increase in body weight is an important sign of a response to therapy. On average hypothyroid patients weigh 15 ± 30% more than during euthyroid state and decrease their weight during replacement therapy, but 17% still maintain their higher body weight, in spite of otherwise adequate substitution. OverweightOverweight is also common (15 ± 17%) among post hyperthyroid patients 2 after successful therapy (thyroidectomy, radio-iodine treatment) on substitution with thyroxine (T4). Obesity syndromes share also several similarities to signs and symptoms of subclinical hypothyroidism. TH in treatment of obesityNot surprisingly, TH though not universally recommended, belongs to the oldest arsenal of anti-obesity preparations. Desiccated thyroid, thyroglobulin, USP and the colloid protein of the thyroid gland were very popular during the forties. They were used together with diuretics during the ®fties and together with amphetamine and amphetamine derivates during the sixties. TH therapy continued later with synthetic T4 and more recently with T3 and T2.Thyroid hormone is one of the most commonly prescribed medications in Western countries. In the USA in the national prescription audit, synthroid (L-thyroxin, Boots Pharmacy Inc) was ranked ®fth among new and repeat prescriptions.Is the use of TH in obesity treatment scienti®cally warranted?Hyperphagia and overfeeding even in a limited form (as one big meal) leads to increased activity of sympathetic nervous system (SNS) and to a concomitant activation of 5-monodeiodinase 1, respon...