ABSTRACT
Thyroid-stimulating hormone (TSH) controls the synthesis and release of thyroxine (T4) by the thyroid gland. TSH secretion by the pituitary is under the control of hypothalamic thyrotrophin-releasing hormone (TRH), and is inhibited by somatostatin and dopamine. Tri-iodothyronine (T3) is more metabolically active than T4. Some T3 is synthesied by the thyroid gland, but most is produced by conversion of T4 to T3 in peripheral tissues. T4 is bound in plasma to thyroidbinding globulin (TBG), and T3 is bound to TBG and albumin. The metabolically active free (unbound) T4 and T3 concentration can now be measured directly by radioimmunoassay and account for only a small percentage of the total plasma concentrations. T3 andT4 modulate many physiological processes including thermogenesis and growth factor production. They also potentiate the actions of catecholamines and have widespread functions in the development of the CNS.
