The background of the study. Measurement of serum thyrotropin (TSH) is the best test of thyroid dysfunction, but there has been debate about the definition of the normal reference range. This study was done to define the normal range for serum TSH in carefully selected normal subjects.
How the study was done. The study group consisted of self-reported healthy subjects, aged 17 to 66 years, who were recruited from a cohort of monozygotic and dizygotic same- and opposite-sex twin pairs. Among them, 90 were taking some medication, 105 had a family history of thyroid disease; 129 had high serum concentrations of antithyroid peroxidase (anti-TPO) antibodies, antithyroglobulin (anti-Tg) antibodies, or TSH-receptor (TSHR) antibodies; 87 had high serum concentrations of two or three of the antibodies; 34 had a family history of thyroid disease and a high serum concentration of one or more of the antibodies; and 987 (460 women, 527 men) had none of these characteristics.
The results of the study. The reference interval for the 987 subjects with no risk factors for thyroid disease was 0.58 to 4.1 mU/L (95 percent confidence interval, 0.55 to 0.61 mU/L for the lower limit, and 3.8 to 4.2 mU/L for the upper limit). The distribution of the values in the women and men and in the different types of twin pairs was similar to that in all the subjects. The distribution of the values was skewed to lower values in the 105 subjects with a family history of thyroid disease, and that in the 76 subjects with high serum anti-TPO antibody values was skewed to higher values.
The conclusions of the study. Serum TSH concentrations in a large group of normal adult men and women ranged from 0.58 to 4.1 mU/L, similar to the reference ranges based on results from fewer, less carefully selected subjects.
The original article. Jensen E, Hyltoft Petersen P, Blaabjerg O, Hansen PS, Brix TH, Kyvik KO, Hegedus L. Establishment of a serum thyroid stimulating hormone (TSH) reference interval in healthy adults: the importance of environmental factors, including thyroid antibodies. Clin Chem Lab Med 2004;42:824-32.