Context: Patients with type 1 diabetes (T1D) have an increased risk of autoimmune thyroiditis (AIT).
Objective: Our objective was to determine whether levothyroxine (l-T(4)) treatment prevents the clinical manifestation of AIT in euthyroid subjects with T1D.
Design and setting: We conducted a prospective, randomized, open, controlled clinical trial at six tertiary care centers for pediatric endocrinology and diabetes.
Patients: Of 611 children and adolescents with T1D, 89 individuals (14.5%) were identified with positive thyroid peroxidase antibodies (TPOAb), thyroglobulin antibodies (TgAb), or both. Of these, 30 patients (age, 13.3 +/- 2.1 yr) met the inclusion criteria and were randomized to receive l-T(4) (n = 16 patients) or no treatment (n = 14 patients).
Intervention: l-T(4) (1.3 microg/kg daily) was given for 24 months in the treatment group, followed by an additional observation period of 6 months in both groups.
Main outcome measures: Thyroid gland volume (as determined by ultrasound), serum levels of TSH, thyroid hormones, TPOAb, and TgAb were assessed every 6 months for 30 months.
Results: Mean thyroid volume decreased in the treatment group after 24 months (-0.60 sd score) and increased in the observation group (+ 1.11 sd score; P = 0.0218). Serum thyrotropin, free T(4), TPOAb, and TgAb levels were not significantly different in both groups during the entire study period. Hypothyroidism developed in three individuals treated with l-T(4) and in four untreated patients (conversion rate, 9.3% per year).
Conclusions: In this study in euthyroid patients with AIT and T1D, l-T(4) treatment reduced thyroid volume but had no effect on thyroid function and serum autoantibody levels.