Thyroid Thyroid Hormones Are Stimulated By Tsh From The Anterior

Thyroid Thyroid hormones are stimulated by TSH from the anterior pituitary, which binds to a TSH receptor and causes a cascade of events that convert tyrosine bound to a thyroglobulin molecule. Iodine is added forming either MIT (where there is only 1 iodine) or DIT (Where there have been two ionisations. These two molecules are coupled by thyroid peroxidise to form either T4 (DIT + DIT), T3 (MIT +DIT) or reverse T3 (DIT + MIT). T4 is more resistant to degredation with a half life of about 7 hours, while T3 has a half-life of less than one day. However T3 is better in thyroid hormone actions than T4, so more T4 is produced by the thyroid (about 70ug are produced daily), which is then converted to T3 as required. Only 5ug of T3 are produced from the thyroid gland but then there is extrathyroidal conversion sites like the liver and kidneys. Hence the T4 is a reservoir, which can be quickly converted by deodinases into either T3 or rT3. The reverse T3 is biologically inactive. The main receptor binding protein of thyoid hormones is Thyroid binding globulin, which is increased in pregnancy, when a patient is taking an oral contraceptive pill or hormone replacement therapy. Upregulation is also due to hepatitis and is highly expressed in newborns. It is downregulated by androgens and anabolics, high density glucocorticoids, chronic liver disease, severe illness, active acromegaly, nephropahy. However this protein is also under genetic control. Other binding proteins include transthyretin and albumen. T3 and T4 has ubiquitous effects and are i important for the growth of the CNS especially oin the 1st years of life. In fact is there isn’t enough thyroid hormones, cretinism : a form of mental retardation, results. T3 and T4 produces calorigenesis or heat production in fact people suffering from hypothyroidism are cold intolerant while hyperthyroidism causes heat intollerence. Thyroid hormone increase the metabolic rate and hence stimulates oxygen consumption, tghe Na+/K+ ATPase, stimulation of mitochondrial oxidative phosphorylation uncoupling, concentration and activation of enzymes, metabolism of substrates, vitamins and minerals, protein synthesis and degredation, enhance ephinephrine’s actions in stimulating glycogenolysis and gluconeogenesis. Thyroid hormones also potentiate the actions f insulin and increase the rate f absorption of glucose uptake by adipose tissue and musc;es. As a result hypothyroidism will result in slow metabolim, slow reflexes bradycardia, and constipation. The opposite takes place with hyperthyroidism.