Thyroid Physiology Flashcards
What shape is the thyroid?
It is a butterfly-shaped gland with 2 lobes with an isthmus in between
What are the 3 hormones that the thyroid produces called?
- Thyroxine (T4)
- Triiodothyronine (T3)
- Calcitonin
What does T3/T4 do?
They increase your metabolic rate
What does calcitonin do?
It contributes to controlling the levels of calcium and phosphorus in the blood
What is the histology of the thyroid gland like?
It is made up of Follicles, which are sphere shaped cells lined with columnar epithelium which is made up of a single layer of thyroid follicular cells that make thyroglobulin. This is the protein that generates the precursors for thyroid hormones.
Within the follicle are colloids, which is the reservoir which the thyroid hormones are made from
What is a tyrosine molecule made from?
Thyroglobulin protein + Iodine
Where is thyroglobulin synthesised? Under the control of what?
Thyroid follicular cells make thyroglobulin under the control of TSH, activated by TSHR, and secretes it into the colloid.
How does iodine get transported into the thyroid?
Iodine gets trapped by the sodium-iodide symporter (NIS) on the surface of the thyroid follicular cells, and also gets transported into the colloid.
What role does thyroid peroxidase have in synthesising thyroid hormones? Where is it found?
It is found on the luminal membrane of thyroid follicular cells, and it iodinates the thyroglobulin
How does T4 travel in the blood?
It binds to the thyroid-binding-globulin in the blood, allowing it to travel around the body.
What happens to T4 once it is in the periphery?
It gets deiodinased into T3, which is the active hormone.
Outline the pituitary-thyroid hormone axis.
- Hypothalamus releases thyrotropin-releasing hormone (TRH)
- Causes anterior pituitary to release thyroid stimulating hormone (TSH)
- This stimulates the thyroid gland to release T3 and T4
Outline the feedback system in the pituatory-thyroid hormone axis.
T3 and T4 are sensed by the hypothalamus and anterior pituitary, and it suppresses the release of TRH and TSH (negative feedback).
Why is free T4 measured instead of total T4?
Pregnancy and the oral contraceptive pill raises thyroid binding globulin, giving high levels of total T4.
