Mechanism of Action
-
Prevention of the stimulant effect of TSH on the adenyl cyclase enzyme.
-
inhibits release of TSH lead to decrease size & vascularity of gland
-
inhibits the release of thyroid hormone by inhibiting proteolytic enzyme, which release T4 and T3 from thyroglobulin (major action).
-
inhibits organic iodine formation
Pharmacological Effects
It improves the manifestation of hyperthyroidism by decreasing the release of T4,T3 from the gland and decreasing the size and vascularity of the gland.
Their effect starts rapidly within 2-7 days, so used in thyroid storm and its effects ended within 10-15 days.
With continued treatment the beneficial effects wear off and manifestations of hyperthyroidism reappear. (iodine escape),i.e. the gland will escape from the iodide block in 2-4 weeks and starts to uptake iodine and form T3, T4 and may produce severe exacerbation of thyrotoxicosis.
Indications
-
Treatment of hyperthyroidism: it is given with thiouracils to decrease size and vascularity the gland.
-
Treatment of hyperthyroid storm. Preoperative preparation of the patient before thyroidectomy to improve the condition and to decrease size and vascularity of the gland.
Disadvantages
It increases intraglandular stores of iodine, which may delay onset of thiouracil therapy or prevent use of radioactive iodine therapy for several weeks.
- Iodine escape (in 2 - 4 weeks).
- It produces fetal goiter if it is used during pregnancy.
Side EffectsY
- Hypersensitivity reaction (drug fever, rash or rarely anaphylaxis).
- Nausea, vomiting, diarrhoea and metallic taste.
- Swollen salivary glands, mucus membrane ulcerations.
- Conjunctivitis, rhinorrhoea
Contraindication and PrecautionsY
- It can not be used alone and/or for prolonged treatment of hyperthyroidism due to iodine escape phenomena.
- It is not given with perchlorates because iodides prevent its action.
- If it is used in association with thiouracil, it should be initiated after onset of thiouracil therapy.
- It should be avoided with or immediately before radioactive iodine because the gland in this case is fully saturated with iodine.