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Thyroid and Antithyroid Drugs. By: Bohlooli S, PhD School of Medicine, Ardabil University of Medical Sciences (ArUMS). THYROID PHYSIOLOGY. Iodide Metabolism The recommended daily adult iodide (I-) intake is 150 mcg Biosynthesis of Thyroid Hormones Transport of Thyroid Hormones
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Thyroid and Antithyroid Drugs By: Bohlooli S, PhD School of Medicine, Ardabil University of Medical Sciences (ArUMS)
THYROID PHYSIOLOGY • Iodide Metabolism • The recommended daily adult iodide (I-) intake is 150 mcg • Biosynthesis of Thyroid Hormones • Transport of Thyroid Hormones • thyroxine-binding globulin (TBG) • about 0.04% of total T4 and 0.4% of T3 exist in the free form.
Evaluation of Thyroid Function • THYROID-PITUITARY RELATIONSHIPS • AUTOREGULATION OF THE THYROID GLAND • ABNORMAL THYROID STIMULATORS
THYROID HORMONES • Chemistry • Are levo (L) isomers • Dextro (D) isomer of thyroxine, dextrothyroxine, has approximately 4% of the biologic activity of the L isomer • Pharmacokinetics • Mechanism of Action
Effects of Thyroid Hormones • The thyroid hormones are responsible for : • Optimal growth • Development • Function • Maintenance of all body tissues
Thyroid Preparations • Synthetic: • levothyroxine, liothyronine, liotrix • Animal origin • desiccated thyroid
ANTITHYROID AGENTS • Thioamides • Anion Inhibitors • Iodides • Iodinated Contrast Media • Radioactive Iodine • Adrenoceptor-Blocking Agents
Thioamides • Pharmacokinetics • Propylthiouracil is rapidly absorbed and the bioavailability is 50-80% • Methimazole is completely absorbed • Both thioamides cross the placental barrier
Thioamides; Pharmacodynamics • Act by multiple mechanisms • Prevent hormone synthesis by inhibiting the thyroid peroxidase • Inhibiting the peripheral deiodination of T4 and T3 • The onset of these agents is slow, often requiring 3-4 weeks
Thioamides: Toxicity: • Occur in 3-12% of treated patients • Nausea and gastrointestinal distress • The most common adverse effect is a maculopapular pruritic rash (4-6%) • Rare adverse effects include: • an urticarial rash, vasculitis, a lupus-like reaction, lymphadenopathy, hypoprothrombinemia, exfoliative dermatitis, polyserositis, and acute arthralgia. • Hepatitis can be fatal • The most dangerous complication is agranulocytosis
Anion Inhibitors • Perchlorate (ClO4-) • Pertechnetate (TcO4-) • Thiocyanate (SCN-)
Iodides • Inhibit organification • Hormone release • Decrease the size and vascularity of the hyperplastic gland
Iodinated Contrast Media • diatrizoate • iohexol • inhibit the conversion of T4 to T3 • inhibition of hormone release
Radioactive Iodine • 131I is the only isotope used for treatment of thyrotoxicosis • therapeutic effect depends on emission of rays
Adrenoceptor-Blocking Agents • Metoprolol • Propranolol • Atenolol • effective therapeutic adjuncts in the management of thyrotoxicosis
CLINICAL PHARMACOLOGY OF THYROID & ANTITHYROID DRUGS HYPOTHYROIDISMHYPERTHYROIDISM Thyroid Storm OphthalmopathyDermopathy