The thyroid gland Flashcards
What are the thyroid hormones
T3 - triiodothyronine
T4 - thyroxine
What are the two cell types in the thyroid gland
C (clear) cells
Follicular cells
Follicular cells are more abundant
What do C (clear) cells secrete
Calcitonin which has a role calcium regulation
What do follicular cells do
Support thyroid hormone synthesis
Describe the structure of thyroid follicles
Spherical with walls made up of follicular cells - the centre of the follicle is filled with a sticky glycoprotein matrix called colloid - The thyroid hormones found in the colloid are in a precursor inactive form
What is thyroid peroxidase and what does it do
An enzyme which is exocytosed into the colloid along with thyroglobulin which catalyses the oxidation of iodide to iodine and the addition of iodine to tyrosine on the thyroglobulin molecule
What does one monoiodotyrosine and one diiodotryosine form
triiodothyronine (T3)
What do two diiodotyrosine form together
Thyroxine (T4)
What is the function of TSH
It is released from the pituitary gland and is a trophic hormone so causes the release of thyroid hormones
Are T3 and T4 lipid soluble and how do they travel around the body
yes - they are amine hormones but behave as steroid hormones as they bind to plasma proteins and circulate int he plasma
Why does T4 have a longer half life than T3
Thyroxine binding globulin has a higher affinity for T4 than T3
What type of hormone can exert negative feedback loop on TSH and TRH
T3 and T4 which is free in the plasma and not bound
What kind of hormone is physiologically active
Free hormone
Why is T3 more physiologically active than T4
Because the thyroid hormone receptors have a higher affinity for T3 than T4
How are hypothyroid patients supplemented with only T4 but have normal T3 levels
T4 is deiodinated into T3 by deiodinase enzymes - half is deiodinated in the plasma and half in the target cell
