Endocrine Abstracts

Background: Excessive iodine exposure can precipitate thyroid dysfunctions through two mechanisms: the Jod-Basedow effect, where failure of inhibition and autoregulation of thyroid gland leads to hyperthyroidism, and the Wolf-Chaikoff effect - a phenomenon in which persistent inhibition of the thyroid gland results in hypothyroidism. Iodinated contrast media and amiodarone are common iatrogenic triggers, and elderly patients and those with underlying thyroid disease are particularly at risk (1–2).

Case 1: An 80-year-old man with ischaemic heart disease, atrial fibrillation, and heart failure presented with diarrhoea and unintentional weight loss. Investigations showed hyperthyroidism (T4 32.3 pmol/l, TSH <0.01 mU/l) with negative antibodies and multinodular goitre on ultrasound. Previous results indicated subclinical hyperthyroidism. Three weeks earlier, he had undergone percutaneous coronary intervention with iodinated contrast. He was diagnosed with Jod-Basedow thyrotoxicosis and commenced on carbimazole with improvement on follow-up. It was noteworthy that he did not exhibit any autonomic signs and symptoms of thyrotoxicosis likely due to regular use of bisoprolol for his atrial fibrillation.

Case2: An 89-year-old man with chronic kidney disease and atrial fibrillation on long-term amiodarone was admitted with worsening renal function and bradycardia with an average heart rate of 50 beats per minute. Investigations revealed profound hypothyroidism (TSH 167.6 mU/l, FT4 4.4 pmol/l) with negative autoantibodies and a normal ultrasound scan of the thyroid gland. This was consistent with amiodarone-induced hypothyroidism demonstrating the Wolf-Chaikoff effect. Amiodarone was discontinued and levothyroxine was initiated, leading to euthyroidism during follow-up.

Conclusion: Iodine-induced thyroid dysfunction is an important but under-recognised complication. These two cases demonstrate that susceptible patient groups, including the elderly population and those with chronic kidney disease and underlying thyroid conditions, are at an increased risk of developing overt thyroid dysfunction following iatrogenic iodine exposure. A detailed clinical history, including recent contrast exposure and medication history, with a low threshold for thyroid function testing, is essential to establish a diagnosis. Prompt recognition allows timely treatment and optimisation of outcomes.

References: 1. Leung AM, Braverman LE. Consequences of excess iodine. Nat Rev Endocrinol. 2014 Mar;10(3):136-42. doi: 10.1038/nrendo.2013.251. Epub 2013 Dec 17. PMID: 24342882; PMCID: PMC3976240. 2. Seo Young Sohn, Kosuke Inoue, Connie M Rhee, Angela M Leung, Risks of Iodine Excess, Endocrine Reviews, 2024;, bnae019, https://doi.org/10.1210/endrev/bnae019