Management of hyperthyroidism during pregnancy: data from a large primary care cohort

An undiagnosed or uncontrolled thyroid disorder can make it harder to conceive and can cause problems during pregnancy. So, it is essential to optimise thyroid status to reduce these risks. Through our 2014 BTF Research Award we helped to fund a study looking at how hyperthyroidism is managed during pregnancy in the UK.

The study results were recently published in the Journal of Clinical Endocrinology and Metabolism. We asked researchers Dr Peter Taylor, Welsh Clinical Academic Trainee, Cardiff University and Dr Bijay Vaidya, Consultant in General Medicine, Diabetes and Endocrinology, Royal Devon and Exeter Hospital about their findings:

Why was the study needed?

It is well established that even modest abnormalities in thyroid function tests during pregnancy are associated with poor health outcomes. Hyperthyroidism is common and women affected are frequently of childbearing age. Hyperthyroidism during pregnancy is associated with unfavourable outcomes if not treated optimally. Antithyroid drugs have potential to cause foetal abnormalities and women who have had definitive treatment, such as total thyroidectomy or radioactive iodine therapy, usually require levothyroxine which requires additional monitoring. It is unclear how the treatment of women diagnosed with hyperthyroidism during preconception impacts their thyroid status if they then become pregnant. Therefore, we were delighted to receive an award from the BTF to study this and are thankful to Caroline Minassian (London School of Hygiene and Tropical Medicine) for her assistance to analyse the data for the study.

What was the aim of the study?

We wanted to determine trends in treating hyperthyroid women both before and during pregnancy in the UK. Our aim was to assess how different treatments for hyperthyroidism before pregnancy increase the odds of having suboptimal thyroid status during pregnancy. We defined suboptimal thyroid status as having a Thyroid Stimulating Hormone (TSH) above 4.0 mU/L or TSH below 0.1 mU/L plus free thyroxine (FT4) above the reference range.

Which groups of women did you study?

We used the Clinical Practice Research Datalink, which is a large database of primary care health records for patients across the UK, to evaluate all females aged 15-45 years with a clinical diagnosis of hyperthyroidism and a subsequent pregnancy record between January 2000 to December 2017. We identified 4,712 pregnancies in women with a previous diagnosis of hyperthyroidism.

The women we studied received one of the following three treatment plans for hyperthyroidism:

•       antithyroid drugs up to or beyond onset of pregnancy
•       definitive treatment with thyroidectomy or radioiodine before pregnancy
•       no treatment at pregnancy onset

What were your findings?

Trends

Between 2000-2008, the use of radioiodine before pregnancy fell from 4.7% to 2.9%. This returned to 4.7% by 2011, then dropped further to 1.8% by 2017. The number of women having thyroid surgery before pregnancy decreased more steadily over the study period, from 19.0% to 11.5% (2000-2017). Women treated with radioiodine or thyroid surgery before pregnancy were less likely to have TSH or FT4 recorded during pregnancy than women who became pregnant on antithyroid drugs (65.3% versus 73.8% had TSH recorded).

Treatments for hyperthyroidism and thyroid status during pregnancy

Overall, in 837 pregnancies (17.8%) women received antithyroid drug treatment. Between 2000-2011, the proportion of pregnant women prescribed carbimazole fell from 10.9% to 8.6%. This gradually increased to 11.5% by 2017. A reverse pattern was seen for the antithyroid drug, propylthiouracil (PTU). 

The antithyroid drug type was switched in 155 pregnancies; 19.8% of the switch occurred in the first trimester.  A higher proportion of women were switched from carbimazole to PTU (32.6% of first trimester carbimazole-exposed pregnancies) than from PTU to carbimazole (6.0% of first trimester PTU-exposed pregnancies). 

Pregnancies with prior radioiodine or thyroid surgery had noticeably increased odds of having suboptimal thyroid status compared with pregnancies starting during antithyroid drug treatment.

What did you conclude?

Overall, our data shows that the management of women with hyperthyroidism who become pregnant is not as it should be and needs urgent improvement. We need better thyroid monitoring and prenatal counselling to optimise thyroid status, reduce foetal drug exposure, and ultimately reduce the risk of adverse outcomes in pregnancy.

What should childbearing women do if they have concerns?

If you are planning a baby, or know you are pregnant, we would encourage you to discuss this with your doctor at the earliest opportunity. They will be able to discuss treatment options with you. You can also find helpful resources about managing thyroid disease in conception and pregnancy on the BTF website.