Thyroxines (T3 and T4) are synthesised in the follicles of the thyroid gland.
The protein substrate for thyroxine synthesis is the intrafollicular colloid called thyroglobulin. Iodide which is actively accumulated by follicular cells is reduced to iodine by peroxidases which reacts with tyrosine residues on the thyroglobulin.
Endocytosis of the thyroglobulin results in the release of small quantities of thyroxines into the circulation. There is a large store of thyroxine in the colloid of a normal thyroid gland.
Thyroxines are protein-bound in plasma. The primary transpory proteins are thyroxine binding globulin (TBG), prealbumin and albmumin.
Protein binding acts to:
Thyroid hormones bind receptors within the nucleus and regulate gene expression by binding of the hormone-receptor complex to DNA.
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