Iodine induced thyrotoxicosis in apparently normal thyroid glands

Two male patients aged 36 and 52 years with thyrotoxicosis revealed a serum T3 of 2.8 and 6.5 nmol/l and a serum T4 of 166 and 238 nmol/l, respectively. Both had been exposed to iodine (2-10 mg daily) for 2-12 months before thyrotoxicosis was diagnosed. Urinary iodine excretion was high, 5000 and 10,000 nmol/24 h (624-1250 microgram). The uptake of 131I in the thyroid glands were low, none had goitre. Their iodine intake was interrupted, urinary iodine excretion gradually decreased, and T3 and T4 in serum concomitantly normalized. They were clinically and biochemically euthyroid 9 and 11 weeks after withdrawal. After 14 and 22 weeks they had normal thyroid uptake of 131I, and thyroid scans showed glands of normal size and configuration. TRH-stimulation and a T3-suppression tests became normal. ESR was not elevated in any of the cases, thyroid antibodies against thyroglobulin and follicular cell microsomes were absent and TSAb was undetectable durng the thyrotoxic stage. Thus no evidence of any pre-existing and/or pre-disposing pathological condition in the thyroid glands were found. The mechanism for the iodine-induced thyrotoxicosis in such cases remains obscure.