Function:

• Secretes two significant hormones; thyroxine (T4) and triiodothyronine (T3)

Thyroid Hormone Metabolism:

• in circulation, 99% of T3 and T4 are bound to albumin
• only unbound or "free" fraction is metabolically active
• small portions of T4 slowly deiodinate to T3
• hormones enter cells by both passive and active mech.
• T3 is about five times more potent in stimulating metabolism than T4
• T4 exerts its effects five times longer than T3

Function:

• free T3 &4 bind to cell receptors and activate or repress transcription and synthesis of proteins and m-RNA
• these alterations in protein synthesis regulate observed effect of thyroid hormones [>metabolic rate, oxidative metabolism]
• decreased circulating free T4 stimulates hypothalamus to release TRH
• TRH stimulates anterior hypophysis to release TSH
• TSH increases all activities of thyroid gland

Thyroid system diagram

Function of Thyroid Hormones

• increase metabolic rate
• growth regulators
• cell differentiation
• oxidative metabolism

In general, thyroid hormones exert effect by binding to nuclear receptors and direct action on the genome to initiate DNA transcription and formation of new proteins

Low Concentrations of Thyroid Hormones:

• retarded growth
• increased sensitivity to cold
• delayed shedding of coat
• limb edema
• decreased food consumption
• fertility problems

Excess Thyroid Hormone

• rapid protein catabolism, protein stores mobilized
• tachycardia
• increased appetite
• weight loss
• excessive sweating
• heat intolerance
• goiter

Control of Free Thyroid Hormone

• Very sensitive mechanism
• increased T4 in circulation decreases secretion of TSH by anterior pituitary.
• feedback mechanism primarily effects pituitary but also acts on hypothalamus and TRH declines

During periods of heavy exercise or stress, thyroid hormone is consumed rapidly.