Thyroid

PHARMACOLOGY The principal pharmacologic effect of exogenous thyroid hormones is to increase the metabolic rate of body tissues. The normal thyroid gland contains approximately 200 µg of levothyroxine (T 4)/g of gland, and 15 µg of triiodothyronine (T 3)/g. The ratio of these two hormones in the circulation does not represent the ratio in the thyroid gland, since about 80% of peripheral triiodothyronine comes from monodeiodination of the outer ring of levothyroxine. Peripheral monodeiodination of levothyroxine at the 5 position (inner ring) also results in the formation of reverse triiodothyronine (r T 3), which is calorigenically inactive. These facts would seem to advocate levothyroxine as the treatment of choice for the hypothyroid patient and to mitigate against the administration of hormone combinations, which, while normalizing thyroxine levels, may produce triiodothyronine levels in the thyrotoxic range. Triiodothyronine (T 3) level is low in the fetus and newborn, in old age, in chronic caloric deprivation, hepatic cirrhosis, renal failure, surgical stress, and chronic illnesses representing what has been called the “low triiodothyronine syndrome.” Pharmacokinetics:...

PHARMACOLOGY The principal pharmacologic effect of exogenous thyroid hormones is to increase the metabolic rate of body tissues. The normal thyroid gland contains approximately 200 µg of levothyroxine (T 4)/g of gland, and 15 µg of triiodothyronine (T 3)/g. The ratio of these two hormones in the circulation does not represent the ratio in the thyroid gland, since about 80% of peripheral triiodothyronine comes from monodeiodination of the outer ring of levothyroxine. Peripheral monodeiodination of levothyroxine at the 5 position (inner ring) also results in the formation of reverse triiodothyronine (r T 3), which is calorigenically inactive. These facts would seem to advocate levothyroxine as the treatment of choice for the hypothyroid patient and to mitigate against the administration of hormone combinations, which, while normalizing thyroxine levels, may produce triiodothyronine levels in the thyrotoxic range. Triiodothyronine (T 3) level is low in the fetus and newborn, in old age, in chronic caloric deprivation, hepatic cirrhosis, renal failure, surgical stress, and chronic illnesses representing what has been called the “low triiodothyronine syndrome.” Pharmacokinetics:...