Background: Thyroid dysfunction and antibodies are increasingly recognized as risk factors during pregnancy. Thyroid function changes during pregnancy and there is a need for gestational age-specific reference intervals for thyroid hormones. The aim of this study was to calculate gestational age-specific thyrotropin (TSH), free thyroxine (fT4), and free triiodothyronine (fT3) reference intervals in an iodine-sufficient thyroid antibody-negative population.
Methods: The study population consisted of a large, prospective population-based cohort, the Northern Finland Birth Cohort 1986 (singleton births, n = 9362), with extensive data throughout gestation. The subjects underwent serum sampling in early pregnancy. Samples were assayed for TSH, fT4, fT3, thyroid-peroxidase, and thyroglobulin antibodies (n = 5805). All mothers with thyroid antibodies or previous thyroid diseases were excluded when calculating gestational age-specific percentile categories for TSH, fT4, and fT3. Also, associations between body mass index (BMI) and thyroid hormones were established.
Results: The upper reference limit for TSH was 2.5 multiples of median (2.7-3.5 mU/L, depending on gestational week). The lower reference limit was as low as 0.07 mU/L. Reference intervals for fT4 rose during early pregnancy and decreased thereafter, ranging between 11-22 pmol/L. Reference intervals for fT3 were uniform throughout gestation, ranging between 3.4 and 7.0 pmol/L. BMI was associated positively with early pregnancy TSH and fT3 concentrations and negatively with fT4 concentrations.
Conclusions: These gestational age-specific reference intervals for thyroid hormones provide a framework for clinical decision making. Overweight and obesity are increasing problems among fertile women and they are associated with possibility of thyroid dysfunction during pregnancy.