Abstract

Thyroid disorders are common in women of childbearing age and are associated with poor pregnancy outcomes. Usual changes in pregnancy and the lack of pregnancy-specific reference ranges make managing thyroid disorders in pregnancy challenging. Our aim was to establish references levels for blood tests that check thyroid function in pregnancy that are specific to each trimester. Another aim was to examine how common abnormal thyroid blood tests are in pregnant women who do not usually have thyroid disorders. This was a study of thyroid function blood tests (TFTs) in pregnant women attending a large maternity hospital. Women with known thyroid disorders, autoimmune disease, recurrent miscarriage, hyperemesis gravidarum and pre-eclampsia were excluded. TFTs were analysed in the CUH biochemistry laboratory using specialist equipment. Trimester-specific reference ranges (2.5th, 50th and 97.5th centiles) were calculated. Three-hundred-and-fifty-one women were included in the analysis. The middle age point for women included in the study was 30 years. Thyroid-stimulating hormone levels showed slightly increasing levels throughout the pregnancy. Free thyroxine (T4) and T3 blood levels decreased throughout pregnancy. We established pregnancy-specific thyroid function reference figures for pregnant women. These figures can be used in clinical practice.