The aim of this study is to evaluate whether subclinical hypothyroidism causes any alteration of the median nerve diameters by using sonography, and whether L-thyroxine treatment can improve the median nerve dimensions. Twenty-two female patients with subclinical hypothyroidism participated in this study. All patients were asked for the manifestations of carpal tunnel syndrome. They were examined and nerve conduction tests were performed. Right and left median nerve measurements (major and minor axis and cross-sectional area) were determined in all patients by high-resolution sonography before and after euthyroidism was achieved with L-thyroxine treatment. All sonographic measurements were within normal limits in 18 of 22 cases. Four patients had increased the measurements of the median nerve, but only 2 cases of 4 had increased cross-sectional area of median nerve and one of them had clinical symptoms simulating carpal tunnel syndrome. None of them has positive clinical signs of carpal tunnel syndrome. Nerve conduction velocities did not indicate carpal tunnel syndrome in patients with subclinical hypothyroidism. L-thyroxine treatment significantly decreased the mean cross-sectional area, minor and major axes of the right and left median nerves. Clinical symptoms were gradually improved. There was no correlation between the measurement of median nerve and thyroid hormones, thyroid-stimulating hormone, body mass index, and age. The duration of treatment with L-T-4 and dose did not correlate with the measurement of the median nerve. This study shows that subclinical hypothyroidism causes some alterations in the median nerve diameters in a few patients. L-Thyroxine replacement in subclinical hypothyroidism decreases the measurements of the median nerve.