Positive Feedback

The plasma proteins that bind thyroid hormones are albumin, a prealbumin called thyroxine-binding prealbumin (TBPA), and thyroxine-binding globulin (TBG). The free thyroid hormones in plasma are in equilibrium with the protein-bound thyroid hormones in the tissues. Free thyroid hormones are added to the circulating pool by the thyroid. It is the free thyroid hormones in plasma that are physiologically active (increasing the metabolic rate) and imbalances in these hormones result in thyroid disease. In thyroid storm, a form of hyperthyroidism, the normal body temperature of 37.5 oC may rise to over 40 oC.

Parathormone and the thyroid hormone calcitonin work antagonistically to regulate the plasma calcium and phosphate levels.

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