Summary: Care must be taken when considering iodine supplementation because it can provoke latent thyroid autoimmunity resulting in hypothyroidism.

A noteworthy study just published in The American Journal of Clinical Nutrition adds more evidence that iodine supplementation, even in small amounts, can produce hypothyroidism. The authors state:

“The beneficial health effects associated with Universal Salt Iodization are well known. Yet, little is known about the possible adverse health effects in people with high iodine intake and the safe daily intake upper limit in the Chinese population…The objective of this study was to explore the safe upper level of total daily iodine intake among adults in China.”

They examined 256 adults with apparently normal thyroid function in a 4 week double-blind, placebo-controlled, randomized controlled trial. The subjects were randomly assigned to 12 different levels of iodine supplementation ranging from 0 to 2000 micrograms per day (2000 μg = 2 milligrams). Iodine from both supplements and diet was taken into consideration. They were then evaluated for thyroid function, thyroid size, and urinary iodine. The outcome was striking for what would seem to be a modest amount:

“The mean iodine intake from the diets and salt intake of the participants were 105 ± 25 and 258 ± 101 μg/d, respectively. In comparison with the placebo group, all iodide-supplemented groups responded with significant increases in median urinary iodine concentrations and in thyroid-stimulating hormone concentration. Thyroid volume decreased after 4 wk in the high-iodine intervention groups (1500–2000 μg). Subclinical hypothyroidism appeared in the groups that received 400 μg I (5%) and 500–2000 μg I (15–47%).”

This is striking in that even 400 micrograms, only 0.4 milligrams, provoked subclinical hypothyroidism in a significant percentage of patients. This is why I published an earlier post regarding the need for care in the use of iodine for radiation protection, to say nothing of the inappropriate supplementation of large amounts of iodine without due care. In this study the highest intervention group which was still only 2 mg per day had noticeable thyroid shrinkage. The authors conclude:

“This study showed that subclinical hypothyroidism appeared in the participants who took the 400-μg I supplement, which provided a total iodine intake of ∼800 μg/d. Thus, we caution against a total daily iodine intake that exceeds 800 μg/d [0.8 milligrams] in China and recommend further research to determine a safe daily upper limit.”

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Summary: Borderline levels of TSH (thyroid stimulating hormone) still within the reference ranges typically printed in laboratory reports can indicate low thyroid function (and predict hyperthyroid on the other end of the scale). A thorough assessment of the more than two dozen patterns of thyroid dysfunction is necessary for an accurate diagnosis.

Clinicians and patients may often be misled by TSH levels that appear normal, but experienced practitioners know that they can mask the presence thyroid disorders. Because hypothyroidism affects function globally, a study just published in the Journal of Clinical Endocrinology & Metabolism that practitioners in all specialties should be vigilant. The authors state:

“Serum TSH in the upper part of the reference range may sometimes be a response to autoimmune thyroiditis in early stage and may therefore predict future hypothyroidism. Conversely, relatively low serum TSH could predict future hyperthyroidism…The objective of the study was to assess TSH within the reference range and subsequent risk of hypothyroidism and hyperthyroidism.”

The authors examined 10,083 women and 5,023 men without previous thyroid disease who had a baseline TSH of 0.20–4.5 mU/liter for the predictive probabilities of developing hypothyroidism or hyperthyroidism according to categories of baseline TSH during follow-up 11 years later. Their data drew a strong result:

“During 11 yr of follow-up, 3.5% of women and 1.3% of men developed hypothyroidism, and 1.1% of women and 0.6% of men developed hyperthyroidism. In both sexes, the baseline TSH was positively associated with the risk of subsequent hypothyroidism. The risk increased gradually from TSH of 0.50–1.4 mU/liter [women, 1.1%; men, 0.3%] to a TSH of 4.0–4.5 mU/liter (women, 31.5%; men, 14.7%). The risk of hyperthyroidism was higher in women with a baseline TSH of 0.20–0.49 mU/liter (3.9%) than in women with a TSH of 0.50–0.99 mU/liter (1.4%) or higher (∼1.0%).”

Too many patients with thyroid dysfunction fall between the cracks of routine care. This evidence strongly supports the importance of a complete assessment of thyroid function when these disorders, especially autoimmune thyroid disease, are suspected. The authors conclude:

“TSH within the reference range is positively and strongly associated with the risk of future hypothyroidism. TSH at the lower limit of the reference range may be associated with an increased risk of hyperthyroidism.”

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This paper published in the medical journal Menopause International touches on the huge topic of thyroid dysfunction before, during and after menopause. As the author states, “Thyroid dysfunction is common, especially among women over the age of 50. In caring for peri- and post-menopausal women, it is important to recognize the changing clinical manifestations of thyroid disease with age.” Subclinical hypo and hyperthyroidism (thyroid dysfunction in the presence of normal TSH levels), an extremely important topic that you will see more about here, is noted in particular. The author notes, “…caution is required in diagnosing and treating thyroid dysfunction in women who are taking oral estrogens or selective estrogen receptor modulators.” The functional approach that fully examines and treats the two dozen underlying patterns of thyroid dysfunction with appropriate tests and therapies is far more extensive than indicated here. See Dr. Kharrazian’s book for an overview for the layperson.

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