Objective: To determine the factors that influence the development of abnormal thyrotropin (TSH) level in an euthyroid population.
Methods: We conducted a follow-up study in 3 communities with different iodine status. Of the 3403 euthyroid subjects at baseline screened in 1999, 80.1% (n = 2727) was visited and sampled in 2004 for measuring TSH, thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb).
Results: Iodine status in the 3 communities were stable. Decreased TSH level (< 0.3 mU/L) developed in 2.5% (n = 68) of sampled subjects, while raised TSH level (> 4.8 mU/L) in 2.4% (n = 64). A logistic analysis showed that risk factors for developing decreased TSH level included positive conversion of TPOAb (OR = 5.5), positive TPOAb both in 1999 and in 2004 (OR = 4.0), positive TgAb in 2004 (OR = 3.7) and TSH < 1.0 mU/L in 1999 (OR = 2.6). Risk factors involved in developing raised TSH level included iodine status of Zhangwu community (OR = 4.1), iodine status of Huanghua community (OR = 3.9), positive TgAb in 2004 (OR = 3.7), positive TPOAb both in 1999 and 2004 (OR = 3.6), positive conversion of TPOAb (OR = 2.7) and TSH > 1.9 mU/L in 1999 (OR = 2.6).
Conclusions: Exposure to long-term iodine excess imposes danger of developing hypothyroidism. The risk will be even higher when exposing to iodine adequacy after correction of iodine deficiency. An interval between 1.0 and 1.9 mU/L of TSH level was optimal with the least probability of developing abnormal TSH level.