Thyroid Hormones

DEFINITION
Serum concentrations of T4, T3, free thyroxine, or endogenous canine TSH outside the normal range

PATHOPHYSIOLOGY
The thyroid gland regulates basal metabolism; two molecules, tyrosine and iodine, are important for thyroid hormone synthesis.
The tyrosyl ring can accommodate two iodide molecules; if one iodide attaches, it is called monoiodotyrosine (MIT); if two iodide molecules attach to the tyrosyl ring, it is called diiodotyrosine (DIT).
Two DIT molecules form T4; one MIT coupled with one DIT molecule forms T3.
T4 is the major storage form of thyroid hormone; T3 is the active form of the hormone; most T3 is formed outside the thyroid gland by deiodination of T4.
Another type of T3 is formed when an iodide molecule is removed from the inner phenolic ring of T4; this compound is called reverse T3 and increases in nonthyroidal illness.
Thyrotropin, or TSH, is the most importantregulator of thyroid activity.
TSH secretion is regulated by thyroid hormones via negative feedback inhibition of the synthesis of TRH at the level of the hypothalamus and by inhibition of the activity of TSH at the level of the pituitary.
With thyroid gland failure, the pituitary gland senses decreases in serum free thyroxine (FT4) and TT4, resulting in increased serum endogenous TSH concentration.
The use of endogenous TSH alone is not recommended as a method of assessing thyroid function.
FT4 concentrations are measured by equilibrium dialysis (gold standard) or analogue immunoassays.