Thyroxine dosage during pregnancy in women with primary hypothyroidism

ABSTRACT

Objective To assess whether pregnancy changes the thyroxine requirements of hypothyroid women.

Design A retrospective, longitudinal study.

Setting Queen Charlotte's and Chelsea Hospital for Women.

Subjects 32 women referred for antenatal care during 35 pregnancies.

Main outcome measures Changes in thyroid stimulating hormone (TSH) and free thyroxine (fT4) levels as pregnancy progresses.

Results In most of the pregnancies (80%), no change in thyroxine dose was required (mean dose 129 μg). The mean TSH levels in early (1.8 mU/1) and in late pregnancy (1.5 mU/1) were unchanged ( P >0.5 ). In the remaining pregnancies, thyroxine dose was increased after the first antenatal clinic appointment, on the basis of thyroid function test results, from a mean of 104 μg to a mean of 172 μg ( P<0.01 ). These women had a mean early pregnancy TSH of 12.3 mU/1, which decreased by 95% to  1.3mU/1(P<0.01 ).

Conclusions Most of the hypothyroid patients presenting to an antenatal booking clinic are well controlled in early pregnancy and will remain so throughout pregnancy. The dose of thyroxine does not need to be changed, and further assessments of thyroid function should not be necessary. It is unlikely that the patients were all 'over-treated' before conception, since they were referred to us by a large number of independent doctors. Women who are under-treated before the pregnancy are likely to require both increased thyroxine dose and further thyroid function assays. They can generally be easily detected, biochemically, at the first hospital visit.