Overview_of_Thyroid_Physiology

Overview_of_Thyroid_Physiology - Overview of Thyroid...

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+Overview of Thyroid Physiology Synthesis of T 4 & T 3 T 4 (thyroxine, tetraiodothyronine) and T 3 (triiodothyronine) are synthesized from thyroglobulin in the follicular cells of the thyroid gland. Thyroglobulin is the storage form of the thyroid hormones and serves as reservoir of thyroid hormone. T 4 and T 3 are iodinated derivatives of the amino acid, tyrosine. The iodination of tyrosine to form monoiodotyrosine and diiodotyrosine, and the coupling reaction to form T 4 or T 3 are catalyzed by the enzyme, thyroid peroxidase. Thyroid peroxidase antibodies (formerly called microsomal antibodies) are present in the autoimmune disorder, Hashimoto’s thyroiditis. The thyroid gland synthesizes and secretes more T 4 (80 – 100 ug) than T 3 (4 ug). Synthroid/levothyroxine is the synthetic (drug) form of T4. Cytomel or liothyronine is the synthetic (drug) form of T3. Thyroid Hormone Transport T 4 is the major form of circulating thyroid hormone (100 nmol/L), whereas T 3 is 2 nmol/L. 99% of thyroid hormone is transported bound to Thyroxine binding globulin (TBG), transthyretin, and albumin. Most is bound to TBG. The bound form of thyroid hormone is inactive and serves as reservoir of preformed thyroid hormone that can buffer changes in circulating thyroid hormone levels. Free (unbound) thyroid hormone accounts for less than 1% of the thyroid hormone found in serum. It is the metabolically active form. Currently, labs are able to measure levels of free T 4 and T 3 , thus reducing the need for measurements of Free T 4 Index, total T 4 and T 3 resin uptake. It is important to note that the serum half life for T 4 is long (7 days). Since it takes 4-5 half lives for a drug to reach a therapeutic plateau level, it will take at least one month to achieve a therapeutic level of thyroid hormone replacement (levothyroxine [Synthroid]) in a person with clinical hypothyroidism. This is the reason for scheduling follow-up visits every 6 weeks to monitor TSH and adjust drug dosages accordingly. Thyroid Hormone Metabolism T 4 is the major thyroid hormone in the blood. 40% of it is converted to T 3 in the blood via Type 1 5’-deiodinase in the thyroid, kidney and liver. This accounts for 80% of the circulating T 3 . T 3 is the more physiologically active (3-5 times more potent) form of thyroid hormone, and is therefore, largely responsible for the cellular and metabolic effects of thyroid hormone. Within the skeletal muscle, CNS, and pituitary gland, Type 2 5’-deiodinase converts T 4 to T 3.
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