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

BACKGROUND: Hypothyroid patients on l-T(4) 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(4) 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 microU/ml) under l-T(4) 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(4) dose. We also studied a group of 60 patients receiving l-T(4) replacement after total thyroidectomy. RESULTS: PCA-positive (155 of 391) and PCA-negative (236 of 391) patients did not differ for pretherapy serum TSH levels and thyroid volume. The l-T(4) requirement was significantly (P = 0.002) higher in PCA-positive (1.24 +/- 0.40 microg/kg x d) than in PCA-negative patients (1.06 +/- 0.36 microg/kg x 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 microg/kg x d) compared with those without gastric damage (1.15 +/- 0.33 microg/kg x d) (P < 0.0001). The increased l-T(4) requirement was confirmed also in PCA-positive thyroidectomized patients (1.81 +/- 0.27 microg/kg x d) compared with PCA-negative thyroidectomized patients (1.52 +/- 0.24 microg/kg x 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).