THYROID HORMONE SECRETION. CONTROL & DIAGNOSIS. THYROID STIMULATING HORMONE (TSH). Also called Thyrotropin. Secreted by the ANTERIOR PITUITARY. A glycoprotein with a m.w: 28,000. Main function : It increases the secretion of both Tri-Iodothyronine & Thyroxine by the Thyroid Gland.
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CONTROL & DIAGNOSIS
Mechanism of Action:
TSH + TSH receptors (a typical G-protein coupled receptor) on the thyroid follicular cell membrane of the Thyroid Gland
Adenylyl Cyclase stimulated
Protein kinase A is activated
Multiple phosphorylations throughout the cell
EFFECTS ON THE THYROID GLAND:
IN SUMMARY: TSH increases all the known secretory activities of the thyroid gland!
MOST IMPORTANT IS PROTEOLYSIS WHICH CAUSES RELEASE OF THE TH INTO THE BLOOD STREAM WITHIN 30 MINUTES!
Mechanism of secretion:
TRH + TRH receptor in the pituitary cell membrane of the Pituitary Gland
Phospholipase C second messenger system activated
Hypothalamic-Hypophysial portal system
Tri-iodothyronine & Thyroxine
BOTH THESE EFFECTS ARE NOT SEEN WHEN THE HYPOTHALAMIC- HYPOPHYSIAL TRACT HAS BEEN CUT SHOWING THAT THESE EFFECTS ARE MEDIATED THROUGH THE HYPOTHALAMUS
1. TESTS RELATED TO HORMONE CONCENTRATION IN BLOOD:
ALL THE ABOVE VALUES ARE RAISED IN HYPERTHYROIDISM & DECREASED IN HYPOTHYROIDISM
2. DIRECT TESTS OF THYROID FUNCTION:
- RADIOACTIVE IODINE UPTAKE TEST (RAIU): A small dose of 123I is given orally & fraction taken up by the thyroid gland after 24 hours is found by taking pictures at 4 & 24 hours to measure its radioactivity.
Normally this value is about 30%.
thyroid gland removal.
3. INDIRECT TESTS OF HOMEOSTATIC CONTROL:
TSH is the Best single test to screen for thyroid disease.
initiation of thyroid replacement, “pituitary reset”.
4. MISCELLANEOUS TESTS: