L-thyroxine requirement in patients with autoimmune hypothyroidism and parietal cell antibodies

Background: Hypothyroid patients on L-T, therapy may require replacement doses exceeding the theoretical needs to normalize serum TSH due to low patient compliance, drugs interference, and malabsorption. Objective: We examined whether autoimmune gastritis might cause increased L-T, requirement in patients with autoimmune thyroiditis receiving L-T-4 replacement. Patients: We studied 391 patients with clinical or subclinical hypothyroidism from autoimmune thyroiditis who had achieved normal serum TSH concentration (0.3-3.0 mu U/ml) under L-T, for at least 6 months. Patients were screened for serum parietal cell antibodies (PCA) as a marker of autoimmune gastritis, and the PCA status was correlated with the L-T, dose. We also studied a group of 60 patients receiving L-T-4 replacement after total thyroidectomy. Results: PCA-positive (1155 of 391) and PCA-negative (236 of 391) patients did not differ for pre-therapy serum TSH levels and thyroid volume. The L-T, requirement was significantly (P = 0.002) higher in PCA-positive (1.24 +/- 0.40 mu g/kg . d) than in PCA-negative patients (1.06 +/- 0.36 mu g/kg - d), and a significant positive correlation was found between L-T-4 requirement and serum PCA levels. Among PCA-positive patients, L-T-4 requirement was even higher in those with proven gastritis (1.52 +/- 0.40 mu g/kg . d) compared with those without gastric damage (1.15 +/- 0.33 mu g/kg . d) (P < 0.0001). The increased L-T-4 requirement was confirmed also in PCA-positive thyroidectomized patients (1.81 +/- 0.27 mu g/kg . d) compared with PCA-negative thyroidectomized patients (1.52 0.24 mu g/kg . d). Independent variables affecting L-T-4 requirement were PCA and serum TSH at diagnosis. Conclusions: Autoimmune gastritis is an additional factor affecting L-T-4 requirement in patients with autoimmune thyroiditis. Serum PCA measurement should be considered in patients with an unexplained high requirement of L-T-4.