Turner syndrome (TS) is caused by a sex chromosome aberration. The aim was to study the prevalence and incidence of thyroid disease in adults with TS. Women with TS (n = 91; mean age, 37.7 +/- 11 yr) were compared with an age-matched female random population sample (n = 228). At baseline, 15 (16%) TS women were treated for hypothyroidism, and elevated serum TSH was found in another eight (9%). As a result, hypothyroidism was more common in women with TS (25%) than in controls (2%; P < 0.0001). Serum free T4 was lower (P = 0.02), and serum TSH was higher (P < 0.0001) in TS women than in age-matched controls. Of all TS women with hypothyroidism, 10 (43%) had an elevated thyroid peroxidase antibody titer vs. 15 (22%) of those without hypothyroidism (P < 0.05), evenly distributed between the karyotype 45,X and mosaicism. A high body mass index, but not a family history or blood lipids, was associated with hypothyroidism in TS. After the 5-yr follow-up, an additional 11 (16%) developed hypothyroidism, of whom four (36%) had elevated thyroid peroxidase. Altogether, 34 (37%) TS women had hypothyroidism after the 5-yr follow-up. Autoimmune hypothyroidism was common, with an annual incidence of 3.2% in TS. Thyroid function should be checked regularly in TS.