Thyroid peroxidase activity in euthyroid and mild hypothyroid patients with Hashimoto's thyroiditis.

Thyroid peroxidase (TPO) activity was measured spectrophotometrically according to Hosoya's guaiacol method. The mean TPO activity in ten patients with Hashimoto's thyroiditis was 19.8 +/- 7.6 (mean +/- SE) in an arbitrary unit, which was not significantly different from the normal value in seven normal thyroid tissues (33.7 +/- 5.4). The ten patients were divided into two groups, euthyroid and mild hypothyroid, on the basis of their basal serum TSH. In the euthyroid group, TPO activity (8.17 +/- 1.3) was significantly less than the normal tissue (p less than 0.01). In the hypothyroid group, TPO activity (27.64 +/- 13.8) was almost similar to the normal tissue. A positive correlation was obtained between TPO activity and serum TSH in ten patients with Hashimoto's thyroiditis (r = 0.85, p less than 0.01). It was concluded that TPO activity is significantly decreased in Hashimoto's thyroiditis even when the thyroid function was still within normal range, but the activity might be restored to normal in hypothyroid patients by the stimulation of elevated TSH.