Trimester-specific reference intervals of thyroid function tests in Turkish pregnants

Physiological changes in hormone levels occur in thyroid gland during pregnancy. Screening of the thyroid hormone levels and determining trimester-specific reference intervals in pregnancy are important. Guidelines recommend the use of trimester-specific reference ranges for each country. The aim of this study is to determine trimester-specific thyroid function reference intervals for pregnancy in Turkish population. In total, 5000 patients were screened out of which 1258 patients have all of fT3, fT4 and TSH levels measured, were included in the study. Patients were grouped as follows: 482 women were in first trimester, 361 women were in second trimester, and 415 women were in third trimester. All analyses were performed by means of chemiluminescence methods (Liaison (R); DiaSorin S.p.A., Saluggia, Italy). The TSH reference intervals were 0.005-3.65, 0.01-3.63, and 0.2-3.46 mIU/L at the first, the second, and the third trimesters of pregnancy, respectively. The fT4 reference intervals were 0.72-1.79, 0.71-1.26, and 0.65-1.12 ng/dL at the first, the second, and the third trimesters, respectively. The fT3 reference intervals were 2.45-4.03, 2.37-3.85, and 2.31-3.77 ng/dL at the first, the second, and the third trimesters, respectively. It is crucial to determine population- and gestational-specific reference intervals for trustworthy treatment of pregnants.