Maternal hypothyroidism complicates 2-5% of pregnancies. Thyroxine (T₄) replacement is known to improve obstetric outcomes (rates of miscarriage, premature labour and low birth weight), although there is no evidence that it modifies neurological development in the offspring.

This descriptive study looks at the thyroxine requirements, trends in Thyroid Stimulating Hormone (TSH) during pregnancy, and the outcomes in a multi-ethnic cohort of 143 women booking at the Royal Free Hampstead NHS Trust between October 2011 and January 2013. They were divided into three groups according to their serum TSH level at booking, with a further sub-analysis depending on their initial T₄ replacement dose. Dose adjustments were made on the basis of serum TSH measurements every 4–6 weeks with the aim of a level close to 2.00 mUnits/L.

Our data show that, generally, the higher the starting dose of thyroxine, the more likely women are to need increases in dose during pregnancy. The higher the TSH at booking, the more likely women are to need several increases in dose during pregnancy (partly because of our policy to increase in no more than 25mcg increments at a time). The presence or absence of TPO antibodies shows no apparent correlation with either of these issues. The co-existence of GDM, however, was significantly more likely the higher the thyroxine dose required. The pregnancy outcomes achieved were uniformly good, apart from three women who delivered before 34 weeks, including one stillbirth at 23 weeks, due to thrombotic thrombocytopenic purpura.