Presentation on theme: "Thyroid gland The normal circulating thyroid hormones are Thyroxine T4 (90%),Triiodothyronine T3 (9%) and rT3 (1%). Reverse T3 (rT3) is biologically inactive."— Presentation transcript:
Thyroid gland The normal circulating thyroid hormones are Thyroxine T4 (90%),Triiodothyronine T3 (9%) and rT3 (1%). Reverse T3 (rT3) is biologically inactive form of T3. Thyrotoxicosis or hyperthyroidism (Graves disease) is the clinical syndrome caused by an excess of thyroid hormones. Hypothyroidism (Hashimoto's thyroiditis) is a clinical disorder in which there is a deficiency of thyroid hormones.
THYROID HORMONES Increases basal metabolic rate. Potentiates brain development. Potentiates the beta effects of catecholamines.
Thyroid gland Metabolism of thyroid hormones : The primary metabolism of thyroxine is deiodination. Deiodination of T4 may occur by monodeiodination of outer ring producing 3,5,3’- triiodothyronine - T3 ( T3 is 4 times more potent than T4) by 5’-monodeiodinase. Deiodination of inner ring produce 3,3’,5’ – reverse triiodothyronine – rT3 (inactive) by 5-monodeiodinase.
Outer ring Inner ring 5’-monodeiodinase.5-monodeiodinase.
Thyroid hormones Metabolism of Thyroid hormones : Drugs like beta blockers, high dose propylthiouracil and steroid inhibit the 5’-deiodinase activity necessary for conversion of T4 to T3 resulting in low T3 and high of rT3.
Thyroid gland Mechanism of action of thyroid hormones : T4 dissociate from thyroxine binding globulin in plasma before entry into cells. In the cells, T4 is enzymatically deiodinated to T3 which enters nucleus and attaches to specific receptors which promotes protein synthesis.
Anti-thyroid agents THIOAMIDES : Inhibit hormone synthesis by inhibiting peroxidase. Propylthiouracil also inhibits peripheral deiodination of T4 and T3. Methimazole is more potent and longer acting than propylthiouracil. Slow in onset ~ 4 weeks.
Anti-thyroid drugs THIOAMIDES : These are used for treatment of mild thyrotoxicosis and in preparation of surgery. Propylthiouracil is relatively safe and preferred in pregnancy.
Anti-thyroid drugs THIOAMIDES : ADVERSE EFFECTS Common adverse effects includes maculopapular rash, arthralgia and vasculitis. Agranulocytosis – reversible.
Anti-thyroid drugs Potassium iodide : Inorganic iodide It blocks the organification and release through inhibition of proteolysis – known as Wolff – Chaikoff effect. It decrease the size and vascularity – used before surgery. Rapid improvement in 2-5 days. It is an ideal agent for the treatment of severe thyrotoxicosis and preoperatively.
Anti-thyroid drugs Potassium iodide : Inorganic iodide Anti-thyroid effect is not for long term as gland ‘escapes’ from its effect. This is the most effective way of limiting the potential damage to thyroid gland by radiation emergencies.
Anti-thyroid drugs Potassium iodide : Inorganic iodide Chronic use in pregnancy avoided – fetal goiter. Adverse reactions to iodine includes – iodine induced thyrotoxicosis (Jod Basedow’s phenomenon) at low doses. Others includes – salivary gland inflammation and acne.
Anti-thyroid drugs Radioactive Iodine : I-131 is the only isotope used in treatment of thyrotoxicosis while others are used in diagnosis. Administered as sodium I–131 orally. Therapeutic effect depends on emission of beta rays – destroys the thyroid gland. Patients eventually becomes hypothyroid – managed with thyroxine.
Anti-thyroid drugs Radioactive Iodine : I–131 serves as alternative to surgery. Easy, effective, low cost and absence of pain are the advantages. Not advisable for pregnant women.
Anti-thyroid drugs ANION INHIBITORS : Monovalent ions like perchlorate, pertechnetate, thiocyanate can competitively block the uptake of iodine. Anion inhibitors are uncommon in use because of aplastic anemia. These are effective in iodine induced hyperthyroidism
Anti-thyroid drugs Iodinated contrast media : Diatrizoate / Iohexol : They are valuable in hyperthyroidism and as adjunctive in thyroid storm. They inhibit the peripheral conversion of T4 into T3. Inhibition of hormone release is an additional mechanism.
Anti-thyroid drugs Other Anti-thyroid drugs : Propranolol is used in the management of cardiac symptoms of thyrotoxicosis. Lithium is known to inhibit synthesis and release of thyroid hormones. Amiodarone can also result in hypothyroidism.
Thyroid gland The extreme manifestations of untreated hypothyroidism is myxedema coma with a mortality ~ 50%. Myxedema coma is treated with intravenous T4 or sometimes T3. Levothyroxine T4 is the best choice for replacement therapy. Thyroid hormones replacement is monitored by plasma TSH.
Thyroid gland Thyroid storm: Thyrotoxic crisis, is an acute, life-threatening state induced by excessive release of thyroid hormones. Propranolol to minimize cardiac symptoms High-dose propylthiouracil is preferred because of its ability to inhibit peripheral conversion of T4 to T3 Potassium iodide used to block the release of thyroid hormones