 Thyroid hormones are synthesized in the thyroid gland.  Iodination and coupling of two molecules of tyrosine.  Monoiodotyrosine and diiodotyrosine.  T3 and T4 are formed, incorporated in thyroglobulin in the colloid.  Thyroglobulin is taken by the follicular cells, proteolytic enzymes break it and T3 and T4 released into the circulation.

 TSH controls the whole process.  More than 99% of plasma T3 and T4 are bound to thyroid binding globulin (TBG) and albumin.  The free is the active form.  Deiodination of T4 peripherally produces T3.  Some rT3 is also formed (inactive).  T3 is more active than T4.

 Affect metabolic processes  Increases oxygen consumption  Change the expression of some genes.  Essential for normal growth, mental development and sexual maturation.  Increase the sensitivity to catecholeamines.

 T4 inhibits TSH.  TRH stimulates TSH secretion.

 Free T4 vs total  Free T3 (hyperthyroidism)  TSH

 Increased :  High estrogen  Inherited excess  Decreased:  Severe illness  Loss of proteins  Androgens  Inherited deficiency

 Autoimmune  Toxic nodules  Ingestion of thyroid hormones

 Increased metabolic rate

 High T3 and T4  Low TSH

 Primary, Congenital, autoimmune, overtreatment of hyperthyroidism  Secondary, pituitary disease

 Goiter  Slow metabolic rate

 Primary: low T4, high TSH  Secondary: Low T4, low TSH  Low HB  Cholesterol  CK

 Thyroxine

 Congenital hypothyroidism  Short stature  Mental retardation  Preventable  Screen by TSH from cord or heel blood.