THYROID PHYSIOLOGY


 

I. Thyroid hormones are both iodinated derivatives of tyrosine

  1. T3 (triiodothyronine), T4 (thyroxine)
  2. T4 is present in ten times the concentration of T3 in the circulation but tissues bind T3 more avidly
  3. T4 is about 50% in plasma and 50% in tissues; T3 is about 15% in plasma and 85% in tisssues
  4. 80% of T3 in plasma and tissues is made from extrathyroidal de-iodinization of T4; this occurs in circulation and in tissues
  5. 99% of ciculating T3 and T4 are bound to serum proteins made by liver
  1. Thyroxine-binding globulin (TBG)-binds most of circulating T4; see below for factors which affect TBG synthesis
  2. Throxine-binding prealbumin (TBPA)-binds T4 but not T3
  3. Serum albumin-binds the rest
II. Regulation
  1. TRH-a tripeptide, secreted by hypothalamus; stimulates secretion of both TSH and prolactin, probably in a tonic, constant kind of way
  2. TSH-secreted by ant. pit. in reponse to TRH
  1. Glycoprotein made up of one "alpha" and one "beta" subunit;alpha unit is homologous to that of LH and hCG
  2. T3 and T4 block TRH action on ant. pit
  3. Stimulates thyroid iodine uptake, anabolic metabolism of thyroid follicle cells, and secretion of T3 and T4
  4. Goes up in response to decreased thyroid hormone production, down in response to increased production
III. Actions of T3 and T4
  1. Necessary for mental and physical development
  2. In adult, for metabolic regulation
  1. Increase oxcygen consumption and BMR
  2. Increase cardiac output, heart rate
  3. Increases 2,3-DPG levels
  4. Increases bone turnover
  5. Stimulate protein synthesis
  6. Unknown why cause seemingly adrenergic effects