Background: Hypothyroid patients on L-T4 therapy may require replacement doses exceeding the theoretical needs in order to normalize serum TSH, due to low patient compliance, drugs interference and malabsorption.

Objective: We examinated whether autoimmune gastritis might cause increased L-T4 requirement in patients with autoimmune thyroiditis receiving L-T4 replacement.

Patients: We studies 391 patients with clinical or subclinical hypothyroidism from autoimmune thyroiditis who had achieved normal serum TSH concentration (0.3–3.0 µU/ml) under L-T4 since at least six 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-T4 dose. We also studied a group of 60 patients receiving L-T4 replacement after total thyroidectomy.

Results: PCA positive (155/391) and PCA negative (236/391) patients did not differ for pre-therapy serum TSH levels and thyroid volume. The L-T4 requirement was significantly (p=0.002) higher in PCA/positive (1.24±0.40 µg/kg/day) than in PCA/negative patients (1.06±0.36 µg/kg/day) and a significant positive correlation was found between L-T4 requirement and serum PCA levels. Among PCA/positive patients, L-T4 requirement was even higher in those with proven gastritis (1.52±0.40 µg/kg/day) compared with those without gastric damage (1.15±0.33 µg/kg/day) (p<0.0001). The increased L-T4 requirement was confirmed also in PCA/positive thyroidectomizated patients (1.81±0.27 µg/kg/day) compared to PCA/negative thyroidectomizated patients (1.52±0.24 µg/kg/day). Independent variables conditioning L-T4 requirement were PCA and serum TSH at diagnosis.

Conclusions: Autoimmune gastritis is an additional factor conditioning L-T4 requirement in patients with autoimmune thyroiditis. Serum PCA measurement should be considered in patients with unexplained high requirement of L-T4.