After thyroidectomy there is an appreciable incidence of hypothyroidism as judged by FT4I estimates. Pharmacological doses of iodine (10-300 mg/day) usually suppress, whereas physiological doses of iodine (< 5 mg/day) have been reported to both decrease and increase thyroid function. The value of iodine supplementation in preventing post-thyroidectomy hypothyroidism was assessed in a prospective randomised trial. A series of 55 patients with a TSH > 6 mU/l 1 month after bilateral subtotal thyroidectomy or unilateral lobectomy for benign disease were randomised to receive either chloroform water 5 ml/day (placebo) or chloroform water 5 ml/day with 1 mg of iodine to be taken for 20 weeks. With placebo, 62% of bilateral subtotal thyroidectomies were euthyroid at 6 months on no thyroid replacement, while with iodine all were hypothyroid as judged by FT4I. After bilateral subtotal thyroidectomy, the recovery of remnant function is delayed by an iodine supplement of 1 mg/day.