The Frequency of Thyroid Autoantibodies is Higher in the Upper Range of Normal Thyrotropin Values

If the upper bound of serum thyrotropin (TSH) reference range is decreased from 5.0 to 3.0 mIU/L, a large number of patients in a tertiary care practice setting would be reclassified from “normal” to having “biochemical” hypothyroidism. The predictive value of very subtle, hypothyroidism is unknown. In this paper we investigated the frequency of positive thyroid autoantibodies in the upper range of normal TSH values in the general population. Materials and Methods: Between December 1999 and September 2000, 1434 people aged >20 years selected from families living in Tehran District13 were entered for evaluation. Serum TSH (normal range: 0.3-3.5 mIU/L), antithyroperoxidase (TPOAb) and antithyroglobulin (TgAb) antibodies were measured. “Reference sample” was defined by excluding those with a personal history of goiter, hypothyroidism, thyrotoxicosis, thyroid nodule, neck surgery or radiotherapy, hospital admission or taking of thyroid or antithyroid drugs, estrogens or androgens, and lithium during the previous month, pregnant women, abnormal TSH values, and TPOAb+ or TgAb+. Results: Mean serum TSH was significantly lower in the “reference sample” as compared to the total population. The frequency of TPOAb+ and TgAb+ was significantly higher in thosewith TSH values of 2.6-3.5 mIU/L as compared to those with TSH 2.5 mIU/L both in men and women. Once sequential rising values of TSH were entered into the binary logistic regression model, TSH values greater than 2.5 mIU/L could predict cases that were positive for TPOAb+ and/or TgAb+. Female sex and goiter were the two other significant predictors.

Conclusion: In the general population studied, there is a significant positive correlation between the frequency of positive thyroid autoantibodies and TSH values in the upper range of normal values. Studies are recommended to demonstrate the possible benefit of monitoring or therapy for those with TSH values in the upper range of normal values and positive thyroid autoantibodies.