Effect of low dose iodide supplementation on thyroid function in potentially susceptible subjects: are dietary iodide levels in Britain acceptable?†

<jats:title>Summary</jats:title><jats:p> <jats:bold> <jats:sc>objective</jats:sc> The aim of the study was to evaluate the risk of exposure to an Increase in dietary Iodide Intake amongst potentially susceptible population groups in Britain.</jats:bold> </jats:p><jats:p> <jats:bold> <jats:sc>design</jats:sc> A randomized controlled trial was performed in healthy women and In women with underlying thyroid abnormalities due to subcllnical Hashimoto's thyroiditis (diagnosed on the basis of antithyroid antibodies) or previous Iodide deficiency of supplementation with 500 μg/day iodide (giving a total Intake of approximately 750 μg/day) for 28 days versus placebo.</jats:bold> </jats:p><jats:p> <jats:bold> <jats:sc>patients</jats:sc> Two hundred and twenty‐five women aged 25–54, randomly selected from a general practice In Cardiff, were screened for thyroid microsomal antibody. Antibody positive women (<jats:italic>n</jats:italic>=20), and antibody negative controls (<jats:italic>n</jats:italic>=30) were recruited Into the trial comparing Iodide and placebo. In addition, groups of patients aged 60–75 randomly selected from the Cardiff practice (<jats:italic>n</jats:italic>=29), an Iodide sufficient area, and a practice in Dowlals (<jats:italic>n</jats:italic>= 35), a previously iodide deficient area, were also enrolled Into the trial.</jats:bold> </jats:p><jats:p> <jats:bold> <jats:sc>measurements</jats:sc> Changes In free thyroxine and thyro‐trophin levels were measured after 14 and 28 days of Iodide supplementation.</jats:bold> </jats:p><jats:p> <jats:bold> <jats:sc>results</jats:sc> All the iodide supplemented groups responded in the same way with a small fall in free thyroxine and rise In thyrotrophin levels (combined fall In free thyroxine 14 days after the start of supplementation –1.22 (95% confidence interval – 0.59 to −1.84) pmoUl and at 28 days – 0.86 (– 0.30 to –1.43) pmol/I and rise in thyrotro‐phin at 14 days 0.55 (0.19 to 0.92) mU/I and at 28 days 0.59 (0.12 to 1.07) mU/I). In two of the iodide supplemented subjects thyrotrophin levels rose above the laboratory reference range and In a further three subjects initially elevated thyrotrophin values Increased further. In contrast, no changes In thyroid function were observed in the placebo treated controls and none developed biochemical hypothyroidism.</jats:bold> </jats:p><jats:p> <jats:bold> <jats:sc>conclusions</jats:sc> Dietary Iodide Intakes of 750 μg/day or more may adversely affect thyroid function, especially in individuals with borderline hypothroidism</jats:bold> </jats:p>