ABSTRACT. There are some reports about the association of autoimmune thyroid diseases with human T-cell leukemia virus type-I (HTLV-I) infection. The objective of this study was to estimate the seroprevalence rates of antithyroid antibodies in HTLV-I carriers and HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients in Mashhad, northeastern Iran, in order to determine any association between HTLV-I infection and Hashimoto’s thyroiditis (HT).

Methods: Forty-six HTLV-I infected patients (24 patients with HAM/TSP and 22 asymptomatic carriers) and 40 HTLV-I seronegative healthy individuals were screened for the presence of thyroid autoantibodies. The diagnosis of Hashimoto’s thyroiditis was based on the presence of positive thyroid autoantibodies (antithyroid peroxidase and/or antithyroglobulin) and at least one of two additional criteria (hypothyroidism and/or goiter). Analysis of the data was done using the Fisher-Exact test and SPSS statistical software version 13.0. Any P value below 0.05 was considered statistically significant.

Results: We found thyroid autoantibodies in 14 (63.6%) of 22 asymptomatic carriers, 6 (25%) of 24 patients with HAM/TSP, and 3 (7.5%) of 40 HTLV-I seronegative healthy individuals. We found Hashimoto’s thyroiditis in 45.4% of asymptomatic carriers, 25% of HAM/TSP patients, and 5% of seronegative healthy individuals. The percentage of patients with Hashimoto’s thyroiditis was significantly higher (P<0.01) than the percentage in healthy seronegative individuals.

Conclusion: This study demonstrates a high prevalence of Hashimoto’s thyroiditis in the HAM/TSP patients and the HTLV-I carriers in Mashhad. Our findings suggest an association between HTLV-I infection and Hashimoto’s thyroiditis in our region.

Keywords. Hashimoto’s thyroiditis • HTLV-1 • Mashhad, Iran • Myelopathy/tropical spastic paraparesis • Thyroglobulin antibodies • Thyroid peroxidase antibodies