Interpretation of in-vitro thyroid function tests during pregnancy

In-vitro thyroid function tests are difficult to interpret in pregnancy because of, among other things, the effect of oestrogens on thyroid binding globulin (TBG) concentrations. In an attempt to clarify the position, serum concentrations of total thyroxine (T4), free T4, TBG, T4/TBG ratio, tri-iodothyronine (T3) and thyroid stimulating hormone (TSH) were measured. Total T4 and TBG concentrations rose to above the non-pregnant reference range by 20 weeks. The T4/TBG ratio fell to hypo-thyroid values by 20 weeks but although the free T4 level was lower in the third trimester compared with values in the first and second trimesters, only a few subjects had hypothyroid values. The TSH values remained unchanged throughout pregnancy. The significance of these changes is discussed and reference ranges for these hormones at each trimester are provided.