Ans. is 'a" i.e.. Propranolol THYROID HORMONESo Both thyToxine (T4) and triiodothyronine (T4) are iodine containing derivatives of thyronine,o Thyronine is a condensation product of two molecules of amino acid tyrosine.Thyroxine (T4)- 3, 5, 3', 5' - tetraiodothyronineT3- 3, 5, 3' triiodothyTonineo Thyroid hormones are synthesized and stored in the thyroid follicles as a part of thvroglobulin molecule,o Synthesis of thvroid hormones1. Iodide uptakeo First step in the synthesis of T3 & T4 is iodide uptake.o Thyroid cells (follicular cells) have active transport system for iodine uptake - Na+ :I- symporter.o This trapping is stimulated by TSH.o The iodide concentrating mechanism (Na+ : I- symporter) is also found in skin, salivary glands, gastric mucosa, intestine, mammary glands and placenta, but uptake in these organs is not stimulated by TSH.2. Oxidation and iodinationo Iodide trapped by follicular cells is transported to colloid.o In colloid it is oxidised by membrane bound thyroid peroxidase to iodinium ion (I+) or hypoiodous acid (HOI) or enzyme - linked hypoiodate (E-OI).o These oxidized forms of iodine combines with tyrosine residues of thvroglobulin to form monoiodotyrosine (MIT) and diiodotyorosine (DIT) (in this reaction no enzyme is required),o These MIT and DIT are attached to thvroglobulin.3. Couplingo Pairs of iodinated tyrosine residues couple together to form T3 and T4.o Coupling is an oxidation reaction and is catalysed by same membrane bound thyroid peroxidase,o Normally more T4 is formed than T3.o Oxidation and coupling are stimulated by TSH.4. Storage and releaseo Thyroglobulin containing MIT, DIT, T3 & T4 is transported into the follicular cells by endocytosis.o Inside the cells thyroglobulin is broken down by lysosomal proteases.o Released T4 & T3 are secreted into the circulation, released MIT and DIT are deiodinated and the iodide released is reutilized.o The uptake (endocytosis) and proteolysis of thyroglobuline are stimulated by TSH.5. Peripheral conversion of T4 to T3o Peripheral tissues, especially liver and kidney, convert T4 to T3.o Target tissues take up Tj from circulation for their metabolic need, except brain and pituitary which take up T4 and convert it to T3 within their own cells.o Propylthiouracil, propranolol, amiodarone and glucocorticoids inhibit peripheral conversion of T4 to T3.