Tyrosine derived from thyroglobulin is combined with iodine to produce T3 & T4 (Thyroxin)

T3 is 5 times more active than T4, though T4 is produced in larger amounts

‘ACTIVITIES OF TSH’

Increase the size & number of thyroid gland cells
Increase iodide binding
Increase the release of thyroglobulin into the colloid of the gland
Increase pinocytosis of colloid by the thyroid cells
Increase hormone production
Increase release of already produced hormone from the bound thyroglobulin into the bloodstream

In bloodstream the hormones are 99% protein bound.

Thyroxin Binding Globulin (TBG) has the greatest affinity; but Albumin has the greatest capacity for binding the hormones. Thyroxine-binding prealbumin (TBPA) also bind them

REGULATION OF HORMONAL ACTIVITY

For regulation of the hormonal levels, the negative feedback is mediated by the unbound free fraction
Stress inhibits production
Warmth decreases production
Cold increases production
Glucocorticoids, dopamine & somatostatin inhibit TSH secretion

Reference: Smith T, Pinnock C, Lin T. Fundamentals of Anaesthesia, 3rd edn. Cambridge: Cambridge University Press, 2009; p. 474 .