Abstract
The hypothalamic-pituitary-thyroid axis is a classical negative feedback loop. It is important for the synthesis and secretion of thyroid hormones namely thyrotropin releasing hormone (TRH), thyroid stimulating hormone (TSH), thyroxine (T4) and tri-iodothyronine (T3) 2 . Low detectable levels of serum T3 stimulate the paraventricular nucleus of the hypothalamus to synthesise and secrete the tri-pepetide TRH. TRH in turn stimulates the thyrotroph cells of the anterior pituitary gland to secrete TSH. This glycoprotein acts on the seven transmembrane G-protein coupled to TSH receptors (TSHR) on the thyroid gland to promote the synthesis and secretion of the prohormone T4 and active hormone T3. Peripheral conversion of T4 to T3 is achieved by type 2 iodothyronine deiodinase enzyme (D2). D2 contributes to 85% of T3 synthesis; 95% of T4 and T3 are primarily bound to thyroxinebinding globulin (TBG). Uptake of free circulating T3 and T4 is determined by three specific cell membrane transporters: monocarboxylate transporters 8, 10 and organic acid transporter protein-1c1. Intracellular levels of iodothyronine deiodinase enzymes type 1 and 2 determine the activity and availability of active T3 2 .
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