Peptide Hormones of the Thyroid and Parathyroid Glands

The thyroid and parathyroid glands are two important endocrine organs that are heavily committed to the biosynthesis of hormones as chemical messengers. The thyroid gland, which surrounds the larynx, has an enormous variety of metabolic functions. It is itself regulated by thyroliberin, which in turn regulates production of thyrotropin (thyroid 

The iodothyronines are very insoluble molecules and are kept in solution by transport proteins. The most important of these is thyroxine binding globulin (TBG), which carries about 65% T and 70% Tj. It is a small (MW 60,000-65,000) glycoprotein consisting of four subunits. It has a single, high-affmity binding site for T, with an estimated of 1.2 x 10 °M. 

Thyroxine binding prealbumin (TBPA) carries about 30% T but no Tj. Its affinity is only of the order of 10 M, but it is much more abundant in semm than is TBG. The amino acid sequence and the structure of this protein are known. Four identical subunits (127 amino acids each) form a prolate ellipsoid. Noncovalent interactions between the subunits form a channel of I nm diameter along the long axis, which has a funnel-shaped opening of 2.5 nm. The T molecule is held in one arm of this channel, binding 

These hormones induce amphibian metamorphosis, the change of a tadpole into a frog—an obviously very complex series of biochemical and developmental reactions. Human fetuses will show skeletal abnormalities as well as neuromuscular and mental retardation if born with an inadequately functioning thyroid gland. 

Thyroid deficiency (hypothroidism) has been connected to cretinism and myxedema. Cretinism occurs when hypothyroid children are born intellectually handicapped, are small, and have coarse hair and thick skin. Myxedema, seen in older hypothyroid people, is characterized by subcutaneous semifluid deposits, causing puffiness of the hands and face. The basal metabolism of these patients is depressed to 30 0% below normal, and their body temperature and pulse rate are also reduced. Women suffering from hypothyroidism may give birth to children afflicted with cretinism. 

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