Effect of iodine restriction on thyroid function in subclinical hypothyroid patients in an iodine-replete area: a long period observation in a large-scale cohort

Abstract

Background: Excessive iodine intake is related to a higher prevalence of hypothyroidism, including subclinical hypothyroidism (SCH), in iodine-replete areas. This study aimed to evaluate the effect of iodine restriction on thyroid function in SCH patients in an iodine-replete area and analyze the relationship between serum thyrotropin (TSH) levels and iodine intake.

Methods: The study consisted of 146 consecutive patients who were diagnosed with SCH at the Samsung Medical Center between 2010 and 2012. Urinary iodine concentration (UIC) was measured in 82 patients. Of these, 20 patients with UIC < 300 μg/L were excluded, and 62 patients with UIC ≥ 300 μg/L were educated about the restriction of iodine-rich foods. Following the first follow-up visit, these patients were divided into two groups based on the UIC level: group A (well-controlled iodine intake, UIC < 300 μg/L, n = 40), and group B (poorly controlled iodine intake, UIC ≥ 300 μg/L, n = 22). The remaining 64 patients did not restrict iodine rich foods (group C). The 82 patients with measured UICs were reevaluated every 3-6 months, and the median follow-up was 13 months (range 3-32 months). Thyroid function and UIC were measured at each visit. The correlation between serum TSH level and UIC was determined for the 82 patients in whom UIC was measured.

Results: Following 3-6 months of iodine restriction, the serum TSH levels significantly decreased in group A (from 9.0 mU/L to 4.7 mU/L; p < 0.01). In addition, the serum free thyroxine (T4) levels in group A significantly increased (from 1.11 ± 0.23 ng/dL to 1.18 ± 0.17 ng/dL; p < 0.05). However, there were no significant changes in serum TSH or free T4 levels in groups B and C. Serum TSH levels significantly correlated with UIC (r = 0.33, p < 0.01).

Conclusion: Iodine restriction may normalize or, at the very least, decrease serum TSH levels in SCH patients, and serum TSH levels are strongly correlated with UIC. Therefore, restriction of iodine intake could be a primary treatment option in SCH patients in an iodine-replete area.