Thyroid stimulating hormone and free thyroxine in pregnancy: expressing concentrations as multiples of the median (MoMs)

Abstract

Background: Thyroid stimulating hormone (TSH) and free thyroxine (FT4) concentrations vary during pregnancy and conventional units can vary between laboratories. Reference ranges are widely quoted but are arbitrary and do not allow for inter-laboratory differences or gestational age. We therefore explored using multiple of the median (MoM) values to overcome these limitations.

Methods: TSH and FT4 concentrations from 16,346 UK and 5500 Italian women less than 16 weeks of gestation collected as part of the CATS study were converted into MoMs. Effects of maternal age, gestational age, maternal weight, smoking, parity and season of blood sampling were analysed and values adjusted for influencing factors. Distributions of adjusted MoMs were determined.

Results: TSH and FT4 (MoMs) significantly reduced the difference between UK and Italian samples (FT4>TSH) compared with conventional units. TSH and FT4 MoMs were statistically significantly influenced by weight, smoking and parity; season also influenced TSH and age influenced FT4. The first and 99th centile MoMs were TSH 0.2 and 4.01, and FT4 0.75 and 1.39.

Conclusion: Use of TSH and FT4 MoMs in early pregnancy allows for systematic differences between laboratories and other factors. Their use indicates high or low levels in a quantitative manner independent of reference ranges.

Keywords: Free T4; MoM; Multiple of the median; Pregnancy; TSH.