46) .WITH REFERENCE TO THYROID HORMONES

[Editor]

a) the circulating hormones include thyroglobulin
b) the receptor sites are intracellular
c) their release is under control of the posterior pituitary gland
d) tri-iodothyronine is mainly protein bound
e) there is an ion pump mechanism in the follicular cells of the thyroid gland

[Editor]

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The principle hormones secreted by the thyroid are thyroxine T4 and tri iodo thyronine T3.Also T4 is converted peripherally to T3. Small amounts of RT3 are found in the thyroid venous blood-the inactive form.T3 is more active than T4.Secretion of thyroid hormones are controlled by anterior pituitary hormone TSH,which in turn is controlled by TRH secreted by the hypothalamus.Control is by a negative feedback mechanism.Both T3 and T4 are bound to plasma proteins.Only free hormones in plasma are active. The plasma proteins are albumin,thyroxinebinding prealbumin(transeryetin ),and a globulin-thyroxine binding globulin.Albumin has the greatest binding capacity for T4.Thyroid binding globulin has the greatest affinity..
Thyroid hormones enter the target cells where T3 binds to the receptors in the nuclei.T4 also binds but not avidly.T3 is more potent as it is less tightly bound to plasma proteins.RT3 is inert. Iodide responsible for thyroid hormone synthesis is actively transported from circulation to the colloid by the iodide pump-Secondary active transport along with sodium.Sodium is pumped back into the interstitium by Na-K ATPase.