Hyperthyroidism following the ingestion of natural health products

Discussion

Iodine is critical to normal thyroid function. It is taken up by transporters on thyroid cells and used to synthesize iodotyrosines, which form fT₄ and fT₃.⁷ To maintain normal thyroid function, the recommended daily intake of iodine for adults is 150 µg/d, with an upper limit of 1100 µg/d.⁸

With exposure to increasing amounts of iodine (eg, through the ingestion of supplements or iodine-containing contrast), the thyroid has intrinsic regulatory mechanisms in place to maintain normal thyroid function. The Wolff-Chaikoff effect describes the immediate intrinsic reduction in thyroid hormone synthesis following a large iodine load, mainly due to impaired organification of iodide.⁷ This effect is typically transient, lasting only a few days in healthy individuals.⁸ Those with a history of thyroid disease (eg, Graves disease, Hashimoto thyroiditis) can be more susceptible to the Wolff-Chaikoff effect, and hypothyroidism can ensue after iodine exposure.

Iodine exposure can also lead to hyperthyroidism (Jod-Basedow phenomenon),⁷ as in the current case. This phenomenon is classically described in individuals who live in iodine-deficient regions,² but it has also been described in those with a history of Graves disease, multinodular goitre, and thyroiditis.^3,8 In a case series from the United States, patients with a history of Graves disease and Hashimoto thyroiditis developed hyperthyroidism after taking over-the-counter iodine supplements.⁴ In another case, a 39-year-old German woman with a history of goitre developed hyperthyroidism after consuming an herbal tea containing kelp for 4 weeks.³

In the current case, our patient had no known history of thyroid disease. Findings of a thyroid ultrasound were normal, and her levels of anti–thyroperoxidase antibodies were normal. Although her level of TBII antibodies was slightly elevated, she had no clinical features of Graves disease. Findings of her radioactive iodine uptake scan showed low uptake, suggestive of thyroiditis. Although exogenous iodine can lead to findings of low uptake on scans, she had not taken iodine in 4 months.

Iodine-induced hyperthyroidism has been less often described in patients without thyroid disease. A 45-year-old woman in Italy who had no underlying thyroid disease developed thyroiditis shortly after starting a kelp-containing diet. Unlike in our case, her hyperthyroidism persisted for 2 months, followed by hypothyroidism and then resolution.⁹ A 72-year-old woman without thyroid disease from the United States also presented with hyperthyroidism after ingestion of kelp tablets during the previous year. As in our case, her thyroid function normalized upon stopping the tablets.¹⁰ In Israel, a 27-year-old-woman presented with hyperthyroidism after ingesting kelp-containing supplements. She also had no known history of thyroid disease. Seven weeks after discontinuing her supplements, her thyroid function also normalized.¹¹

An additional complexity was that our patient was taking biotin (often marketed as vitamin B7 or vitamin H) for hair and skin health, fatigue, and low energy. Although her biotin dose was low and likely did not unduly influence her test results, high doses of biotin can interfere with endocrine testing, including thyroid function tests. Supraphysiologic doses of biotin are being increasingly promoted as a remedy for poor hair and skin health, fatigue, and low energy.¹² Biotin, however, can interfere with endocrine testing, including thyroid function tests. The type of interference can vary depending on the immunoassays used, but most often high-dose exposure (usually > 1.5 mg/d) leads to a false depression of TSH level and a false elevation of fT₃ and fT₄ levels (ie, it mimics hyperthyroidism).¹² Biotin can also cause a false-positive elevation of TBII levels and false reduction in thyroglobulin levels. Patients are typically advised to withhold biotin-containing supplements for 48 hours before thyroid function testing.¹²