3 - thyroid physiology

Question 1

what are the main hormones produced by thyroid gland?

Answer 1

• thyroxine (T4)
• triiodothyronine (T3)
• calcitonin

Question 2

what is structure of thyroid gland cells?

Answer 2

they are follicles lined with cuboidal follicular cells with inside of colloid
- parafollicular cells in between follicles

Question 3

what is the main important feedback mechanism for thyroid?

Answer 3

hypothalamus produces TRH which stimulates anterior pituitary to release TSH which stimulates thyroid gland to make T3 & T4

*too much or too little T3&T4 can impact back on pituitary gland on how much TSH to make

Question 4

what are the steps to thyroid hormone synthesis?

Answer 4

1. thyroglobulin synthesis (in colloid)
2. uptake & concentration of iodine in follicular cells and oxidation of I- to I
3. iodination of thyroglobulin inside colloid
4. attaches to tyrosine so formation of MIT & DIT

• either store in colloid or release

Question 5

what is MIT and DIT in thyroid hormone synthesis?

Answer 5

made when iodination of thyroglobulin

MIT = monoiodotyrosine unit
DIT = diiodotyrosine unit

DIT + MIT = T3
DIT + DIT = T4

Question 6

what is the main hormone secreted by thyroid hormone?

Answer 6

T4 (thyroxine) = 90% secreted
T3 (triiodothyronine) = 10% secreted

*T4 can be converted to T3 in liver or kidney

Question 7

what is meant by T3 is major biologically active thyroid hormone?

Answer 7

it’s basically just meaning that it’s primary hormone that directly interacts with target cell

• T3 is bound less tightly so easier to release

*that’s why T4 can be turned into T3 in liver

Question 8

how are thyroid hormones transported?

Answer 8

by binding to serum proteins - 70% thyroxine binding globulin (TBG), 20% bound to prealbumin (TBPA), 5% bound to albumin

*only unbound can act on cell - free T3

Question 9

what are a couple things that can increase or decrease TBG?

Answer 9

increase TBG (serum protein) - pregnancy, newborn, hepatitis

decrease TBG - severe illness, large dose glucocorticoids

Question 10

what degrades thyroid hormones?

Answer 10

de-iodinases like D1 (liver & kidney), D2 (heart & muscle etc), D3 (fetal tissue, placentya & brain)

T4 →T3 by D1 (makes it more physiologically active)

T3 →D3 →inactive T2