Hyperthyroidism in pregnancy

• Discuss risks with women before they plan a pregnancy, including long-term treatments such as radioiodine treatment or thyroid surgery.
• Women treated with radioiodine should wait at least 6 months before trying for a baby as TRAb levels raise after this treatment.
• Following thyroid surgery,  it is advisable that women have had 2 thyroid blood tests 3 months apart, showing blood tests are within the reference range.
• If TSH-receptor antibody (TRAb) remains raised at around 6 months after definitive treatment, women should consider delaying becoming pregnant. This is because raised TRAb levels can increase the risk of fetal hyperthyroidism, which presents significant risks to the baby.
• Switch to propylthiouracil (PTU) if being treated with carbimazole pre-pregnancy. PTU is safer in conception and early pregnancy.
• Aim for the lowest possible dose of antithyroid drugs.
• FT4 should be in the upper half of the reference range.
• Care should be managed by an endocrinologist or obstetrician.

During pregnancy

• Switch from carbimazole to propylthiouracil (PTU) before 10 weeks of pregnancy.
• Use the lowest possible dose of antithyroid drugs.
• Aim for FT4 in the upper half of the normal reference range.
• Measure TSH/T4 2-4 weekly until 20 weeks. After this, check 4-8 weekly.
• Graves’ disease often gets better in pregnancy. In women who have been euthyroid for 6 months or more on a low dose of antithyroid drug, consider discontinuing antithyroid drug and monitoring closely.
• Consider increasing the interval between tests if antithyroid drugs are stopped.
• Test TSH-Receptor antibodies (TRAb)  in women with a history of Graves’ disease (even if they have had definitive treatment) at booking (12 weeks).
• Test TRAb again at 20 and 28 weeks if it is either 3 x above the TRAb level at 12 weeks, or in women still on antithyroid drugs.
• Perform regular heart rate auscultation and ultrasound scans from 26 to 28 weeks in women who have:
  • Had uncontrolled Graves’ disease during pregnancy.
  • Being treated with antithyroid drug treatment during pregnancy.
  • Had a TRAb level 3 x above the normal level.

After birth

• Graves’ disease may relapse. Distinguish this from postpartum thyroiditis through taking a history, clinical features of Graves’, TRAb testing and isotope scanning.
• Aim to restore thyroid function levels in the non-pregnancy reference ranges.
• Check thyroid function, including TRAb, 6 weeks after birth
• Counsel women that both carbimazole and PTU are safe in breastfeeding but that they may need to split their dose across the day to reduce concentrations of antithyroid drug in the breast milk.

Subclinical hyperthyroidism

What is subclinical hyperthyroidism?

Subclinical hyperthyroidism is defined as having a Thyroid Stimulating Hormone (TSH) suppressed below the normal reference range,  with FT4 and FT3 concentrations within the normal reference range.

• No treatment is required for SCH
• Repeat TSH and FT4 every 2-4 weeks

If overt hyperthyroidism is detected, follow the overt hypothyroidism guidance