Reference intervals for thyroid hormones in pregnant women

To determine the trimester-specific reference range of thyroid function in a single centre in Beijing.

A total of 361 healthy pregnant women and 122 normal non-pregnant women tested for thyroid function in the outpatient clinic of our hospital were selected as the research subjects. After being grouped according to the inclusion criteria, the test values of thyroid function indicators thyroid-stimulating hormone (TSH) and free thyroxine (FT4) were recorded, respectively. According to the method of establishing reference value standards given in the guide, using the 2.5 percentile of the data distribution as the lower limit of the reference value and the 97.5 percentile as the upper limit of the reference value, we established the laboratory thyroid function indicators TSH and FT4 pregnancy-specific reference value range.

The values of pregnancy-specific thyroid function indexes in the first, second, and third trimesters were as follows: TSH (0.02-3.39 mIU/L, 0.03-3.43 mIU/L, 0.27-3.88 mIU/L); FT4 (12.24-20.77 pmol/L, 10.78-20.75 pmol/L, 9.54-16.02 pmol/L). Serum TSH and FT4 levels showed a weak negative correlation throughout pregnancy. The established reference value range was used to evaluate the thyroid function of pregnant women in this study. The overall screening found subclinical hypothyroidism, hypothyroidism, subclinical hyperthyroidism, and the prevalence of hyperthyroidism to be 3.5%, 0%, 2.5%, and 0.3%, respectively.

The reference range specific to pregnancy differs from that recommended by the American Thyroid Association (ATA), affecting the diagnosis and treatment of thyroid disease in pregnant women. To correctly detect and control these diseases, the pregnancy-specific reference must be set up clinically to avoid clinical over-diagnosis and missed diagnosis.