Mechanism of Action
inhibits the oxidation of iodide to iodine by inhibiting peroxidase enzyme and consequently the iodination of tyrosine (iodine organiflcation) is inhibited.
blocks the coupling of iodotyrosines to form iodothyronines.
Propylthiouracil inhibit the peripheral conversion of T4 to T3. they have “slow onset of action” (often after 3-4 weeks). Immunosuppressive effect leading to decrease TRAb levels.
Pharmacokinetics
Carbimazole and methimazole cross the placental barrier and are concentrated by the fetal thyroid gland. Propylthiouracil is preferable in pregnancy because it is more strongly protein-bound and therefore less crossing the placenta . In addition it is not secreted milk.Â
Clinical improvement:
- starts in 1 week
- euthyroid in 6 weeks, stop drug when euthyroid 4-6 months
- 50-70% relapse on stopping drug I.E. SYMPTOMATIC TREATMENT If anti-thyroid drugs are continued for 1-2 years after euthyroid state is reached, ~one-half of patient attain remission.
Indications
- Treatment of hyperthyroidism.
- To prepare the patient for operation (thyroidectomy).
Side Effects
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Agranulocytosis and inhibition of bone marrow. (1/200-500-1000) reversible usually presents w/ acute pharyngitis/ tonsilitis or pneumonia.
prevented by observing its early manifestations e.g. sore throat and doing repeated leucocytic count.
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Increased size and vascularity of the thyroid. The increased size and vascularity of the gland due to (TSH) is prevented by giving small doses of D.thyroxin or KI with thiouracil. It is avoided in pregnancy because it causes teratogenicity in the form of cretinism.  It is avoided in lactation because it may cause myxedema of infant.
anyone with hypothyrodism take LT4 Medication
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Hypersensitivity: Drug fever, rashes,…etc.
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If they are given to pregnant or lactating female resulting in hypothyroidism of infant (Except propylthiouracil).
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Cholestatic jaundice, hepatitis, lymphadenopathy, headache, diarrhoea, oedema of the feet, arthralgia (rare ).