Thyroid Physiology

Question 1

What is the functional unit in the thyroid?

Answer 1

FOLLICLE - follicular cells surround colloid (thyrogobulin) and take in iodine to produce thyroid hormones

Question 2

What are the 6 steps to thyroid hormone synthesis?

Answer 2

1. thyroglobulin is synthesized in rER and secreted into the colloid
2. iodine (from diet) enters the cell from the blood via Na/I symporter
3. iodine exits the cell into the colloid
4. iodine is oxidized by THYROID PEROXIDASE, becoming I2
5. I2 iodinates tyrosyl residues on thyroglobulin, forming monoiodotyrosine or diiodotyrosine
6. two DITs combine to form T4 or MIT combines with DIT to form T3

Question 3

What are the 4 steps to thyroid hormone secretion?

Answer 3

1. thyroglobulin in colloid is endocytosed into follicular cell
2. lysosome fuses with vesicle and thyroid hormones are cleaved from thyroglobulin
3. hormones are released into blood
4. in blood, most thyroid hormones travel bound to a protein, THYROXINE-BINDING PROTEIN

Question 4

What is the active form of thyroid hormones?

Answer 4

free forms not bond to protein are the only (active) forms able to enter cells

• bound to proteins = no entry = inactive

Question 5

What is the most active form of thyroid hormones?

Answer 5

free T3 —> thyroid gland makes more T4 in comparison, so they are deiodinated by deiodinases into T3 in peripheral tissues (liver, kidneys, etc.)

Question 6

What are 6 functions of thyroid hormones?

Answer 6

1. increases metabolic rate (in almost every body system)
2. growth and development
3. metabolism of carbs, proteins, and fats
4. neurologic (perinatal) development
5. thermogenesis
6. autonomic nervous system regulation

Question 7

What are 6 effects of excess thyroid hormones?

Answer 7

1. HR increase
2. heart contractility increase
3. renal blood flow increase
4. GFR increase
5. BP increase
6. SVR decrease (not enough to counteract increased HR and contractility)

• opposite is true for low concentration

Question 8

What is the pathway that leads to thyroid hormone secretion?

Answer 8

• 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

Question 9

What methods of thyroid hormones are measured in clinical small animal medicine?

Answer 9

• T4
• fT4
• TSH