Effect of iodine restriction on thyroid function in patients with primary hypothyroidism

Area of Science:

• Endocrinology
• Thyroidology
• Internal Medicine

Background:

• Dietary iodine intake in Japan is highly variable, ranging from 0.1 to 20 mg/day.
• Iodine excess is a known potential cause of thyroid dysfunction.

Purpose of the Study:

• To assess the frequency of iodine-induced reversible hypothyroidism in patients with primary hypothyroidism.
• To identify clinical factors associated with spontaneous recovery of thyroid function after iodine restriction.

Main Methods:

• Thirty-three hypothyroid patients underwent a 1-2 month period of iodine restriction (avoiding iodine-containing drugs and seaweed).
• Serum thyrotropin (TSH) levels, antibody titers, ultrasonography, (99m)Tc uptake, and non-hormonal iodine levels were measured before and after restriction.

Main Results:

• Serum TSH levels significantly decreased after iodine restriction, with over 63% showing >50% reduction and 33% becoming euthyroid.
• Reversibility of hypothyroidism correlated inversely with (99m)Tc uptake and was associated with higher non-hormonal iodine levels.
• No significant correlation was found between reversibility and anti-thyroid antibody titers or ultrasonography findings.

Conclusions:

• Iodine restriction can lead to recovery of primary hypothyroidism in over half of affected patients.
• The reversibility of hypothyroidism in this cohort was not significantly associated with Hashimoto's thyroiditis but rather with increased (99m)Tc uptake and elevated non-hormonal iodine levels.