Dietary iodine is absorbed by the gastrointestinal tract, converted to iodide ion, and actively trans-ported into the thyroid gland. Once inside, iodide is oxidized back to iodine, which is bound to the amino acid tyrosine. The end result is two hormones— triiodothyronine (T3) and thyroxine (T4)—which are bound to proteins and stored within the thy-roid. Although the gland releases more T4 than T3, the latter is more potent and less protein bound. Of all circulating T 3, most is formed peripherally from partial deiodination of T4. An elaborate feedback mechanism controls thyroid hormone synthesis and involves the hypothalamus (thyrotropin-releasing factor [TRF] and thyrotropin-releasing hormone [TRH]), the anterior pituitary (thyroid-stimulating hormone [TSH]), autoregulation, and the adequacy of iodine intake.
Thyroid hormone (T3) increases carbohydrate and fat metabolism and is an important factor in determining growth and metabolic rate. An increase in metabolic rate is accompanied by an increase in oxygen consumption and CO2 production, indi-rectly increasing minute ventilation. Heart rate and contractility are also increased, presumably from an alteration in adrenergic-receptor physiology and other internal protein alterations, not from an increase in catecholamine concentrations.