an iodine-containing compound secreted by the thyroid gland, predominantly as thyroxine (T4) and, in smaller amounts but four times more potent, triiodothyronine (T3). These hormones increase the rate of metabolism; affect body temperature; regulate protein, fat, and carbohydrate catabolism in all cells; maintain growth hormone secretion, skeletal maturation, and the cardiac rate, force, and output; promote central nervous system development; stimulate the synthesis of many enzymes; and are necessary for muscle tone and vigor. Derivatives of thyronine, T4 and T3, are synthesized in the thyroid gland by a complex process involving the uptake, oxidation, and incorporation of iodide and the production of thyroglobulin, the form in which the hormones apparently are stored in thyroid follicular colloid. After the proteolysis of thyroglobulin, T4 and T3 are released and transported in the blood in strong, but noncovalent, association with certain plasma proteins; T4 accounts for approximately 90% of iodine in circulation, and T3 for 5%. All phases of the production and release of T4 and T3 are regulated by the thyroid-stimulating hormone secreted by the anterior pituitary gland. Production of thyroid hormones is excessive in Graves' disease and toxic nodular goiter (Plummer's disease), diminished in myxedema, and absent in cretinism. The normal 6- to 7-day half-life of T4 in blood is reduced to 3 or 4 days in hyperthyroidism and extended to 9 or 10 days in myxedema. T3 has a normal half-life of 2 days or less and, like T4, is metabolized most actively in the liver. Pharmaceutic preparations of thyroid hormones extracted from animal glands and the synthetic compounds levothyroxine sodium and liothyronine sodium are used as replacement therapy in patients with hypothyroidism. The dosage is initially low and is gradually increased to the optimal level based on the patient's clinical response and tests of the findings on thyroid function studies. Overdosage or a rapid increase in the dosage may result in signs of hyperthyroidism, such as nervousness, tremor, tachycardia, cardiac arrhythmia, and menstrual irregularity.