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L-Thyroxine Requirement in Patients with Autoimmune Hypothyroidism and Parietal Cell Antibodies

Section of Endocrinology and Metabolism, Department of Internal Medicine, Endocrinology and Metabolism and Biochemistry, University of Siena, 53100 Siena, Italy

Address all correspondence and requests for reprints to: Furio Pacini M.D., Section of Endocrinology, Viale Bracci 1, 53100 Siena, Italy. E-mail: pacini8{at}unisi.it.

Background: Hypothyroid patients on L-T₄ 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-T₄ requirement in patients with autoimmune thyroiditis receiving L-T₄ 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 µU/ml) under L-T₄ 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-T₄ dose. We also studied a group of 60 patients receiving L-T₄ 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-T₄ requirement was significantly (P = 0.002) higher in PCA-positive (1.24 ± 0.40 µg/kg · d) than in PCA-negative patients (1.06 ± 0.36 µg/kg · d), 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 · d) compared with those without gastric damage (1.15 ± 0.33 µg/kg · d) (P < 0.0001). The increased L-T₄ requirement was confirmed also in PCA-positive thyroidectomized patients (1.81 ± 0.27 µg/kg · d) compared with PCA-negative thyroidectomized patients (1.52 ± 0.24 µg/kg · d). Independent variables affecting L-T₄ requirement were PCA and serum TSH at diagnosis.

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

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