We aimed to evaluate the prevalence of lactose intolerance (LI) in patients with Hashimoto's thyroiditis (HT) and the effects of lactose restriction on thyroid function in these patients. Eighty-three HT patients taking l-thyroxine (LT4) were enrolled, and lactose tolerance tests were performed on all patients. Lactose intolerance was diagnosed in 75.9 % of the patients with HT. Thirty-eight patients with LI were started on a lactose-restricted diet for 8 weeks. Thirty-eight patients with LI (30 euthyroid and 8 with subclinical hypothyroidism), and 12 patients without LI were included in the final analysis. The level of TSH significantly decreased in the euthyroid and subclinical hypothyroid patients with LI [from 2.06 +/- A 1.02 to 1.51 +/- A 1.1 IU/mL and from 5.45 +/- A 0.74 to 2.25 +/- A 1.88 IU/mL, respectively (both P < 0.05)]. However, the level of TSH in patients without LI did not change significantly over the 8 weeks (P > 0.05). Lactose intolerance occurs at a high frequency in HT patients. Lactose restriction leads to decreased levels of TSH, and LI should be considered in hypothyroid patients who require increasing LT4 doses, have irregular TSH levels and are resistant to LT4 treatment.