Thyroid Physiology Flashcards
What is the functional unit in the thyroid?
FOLLICLE - follicular cells surround colloid (thyrogobulin) and take in iodine to produce thyroid hormones
What are the 6 steps to thyroid hormone synthesis?
- thyroglobulin is synthesized in rER and secreted into the colloid
- iodine (from diet) enters the cell from the blood via Na/I symporter
- iodine exits the cell into the colloid
- iodine is oxidized by THYROID PEROXIDASE, becoming I2
- I2 iodinates tyrosyl residues on thyroglobulin, forming monoiodotyrosine or diiodotyrosine
- two DITs combine to form T4 or MIT combines with DIT to form T3
What are the 4 steps to thyroid hormone secretion?
- thyroglobulin in colloid is endocytosed into follicular cell
- lysosome fuses with vesicle and thyroid hormones are cleaved from thyroglobulin
- hormones are released into blood
- in blood, most thyroid hormones travel bound to a protein, THYROXINE-BINDING PROTEIN
What is the active form of thyroid hormones?
free forms not bond to protein are the only (active) forms able to enter cells
- bound to proteins = no entry = inactive
What is the most active form of thyroid hormones?
free T3 —> thyroid gland makes more T4 in comparison, so they are deiodinated by deiodinases into T3 in peripheral tissues (liver, kidneys, etc.)
What are 6 functions of thyroid hormones?
- increases metabolic rate (in almost every body system)
- growth and development
- metabolism of carbs, proteins, and fats
- neurologic (perinatal) development
- thermogenesis
- autonomic nervous system regulation
What are 6 effects of excess thyroid hormones?
- HR increase
- heart contractility increase
- renal blood flow increase
- GFR increase
- BP increase
- SVR decrease (not enough to counteract increased HR and contractility)
- opposite is true for low concentration
What is the pathway that leads to thyroid hormone secretion?
- hypothalamus is induced by low concentrations of T3 and T4 to produce TRH
- TRH causes the pituitary to produce TSH
- TSH is released and causes thyroid gland to produce and secrete T3 and T4
- high concentrations of T3 and T4 causes a negative feedback loop that blocks TRH production and release
What methods of thyroid hormones are measured in clinical small animal medicine?
- T4
- fT4
- TSH