62 ) THYROXINE

[Editor]

a) is formed from tyrosine
b) is highly protein bound
c) release from the thyroid is inhibited by high levels of tri-iodothyronine
d) causes hypercholesterolaemia in thyrotoxicosis
e) is a steroid hormone

[Editor]

TTTFF

Thyroxine(T4) and tri-iodothyronine(T3) are iodine containing amino acids. Iodination of tyrosine residues of thyroglobulin produces T3 and T4, which are cleaved from the parent molecule. Both hormones are more than 99% bound to plasma proteins including thyroxine-binding globulin and albumin. Control of T3 and T4 production is by TSH secreted by the pituitary gland. Secretion of TSH is inhibited by T3, T4, stress and stimulated by the hypothalamus (via TRH). T3 and T4 are under negative feedback control, both inhibiting the release of TSH and TRH. Thyroid hormones cause a reduction in serum cholesterol concentration as a result of increase in hepatic LDL receptors.