<prompt> Which of the following decreases thyroid hormone on a long term basis ?T4I131CalcitriolFluorouracil  <response>: Ans. is b i.e., I131 Radioactive iodineo Radioactive iodine is used in the treatment of hyperthyroidism.o Radioactive isotopes of medical importance are -I131 half life - 8 days - most commonly used.I123 half life -13 hours.I125 half life - 60 days,o I131 emits X-rays b-rays.o b-rays are utilized for destruction effect on thyroid cells, while X-rays g-rays are useful in tracer studies,o The b-particle penetrate only .5-2 mm of tissue and thus allows therapeutic effects on the thyroid without damaging to the surrounding structure, particularly the parathyroid,o Response is slow - starts after 2 weeks and reaching peak at 3 months,o Uses of radioactive iodineHyperthyroidism due to Graves disease - most common indication.Metastatic carcinoma of thyroid especially papillary or those cases of follicular which concentrate iodine.DosesDiagnostic- 25- 100 curieTherapeutic - 3-6 m curieContraindications1. Pregnant lactating women 2. Young patients. 