Maternal thyroid function at 11-13 weeks of gestation in women with hypothyroidism treated by thyroxine

The aim of this study in pregnant women with hypothyroidism treated by levothyroxine is to examine the interrelations between thyroid-stimulating hormone (TSH), free thyroxine (FT(4)) and free tri-iodothyronine (FT(3)), and examine whether in such patients the treatment is adequate.

This was a retrospective cross-sectional study. Maternal serum concentrations of FT(3), FT(4) and TSH were measured at 11-13 weeks in 164 singleton pregnancies from women with hypothyroidism before pregnancy receiving treatment with thyroxine. The values were compared to the results in 4,318 normal singleton pregnancies.

In the hypothyroid group, compared to the normal group, there was an increase in median TSH (1.990 vs. 1.007 MoM) and FT(4) (1.052 vs. 0.992 MoM) and decrease in FT(3) (0.901 vs. 0.991 MoM). Serum FT(4) was at or above the 2.5th centile in 158 (96.3%) cases but TSH was above the 97.5th centile in 48 (29.3%) and FT(3) was below the 2.5th centile in 49 (29.9%) cases. In both the hypothyroid and unaffected groups there were significant associations between TSH and FT(4), TSH and FT(3) and between FT(3) and FT(4).

In a high proportion of pregnant women with hypothyroidism treated with levothyroxine there is evidence of persistent hypothyroidism because the treatment is inadequate in correcting the levels of FT(3).