Women who have been treated with antithyroid drugs, or who have had hyperthyroidism after a past pregnancy are at greater risk of postpartum hyperthyroidism. Women with Graves' disease have a higher overall relapse rate – 84% -- after pregnancy, compared to a 56% relapse rate in women who have not become pregnant.
Evaluating Postpartum HyperthyroidismAccording to the 2011 "Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum," the challenge of hyperthyroidism that appears after pregnancy is determining if the overactive thyroid is due to postpartum thyroiditis, or Graves' disease. The two conditions require different treatments, and have different outcomes.
To differentiate the two conditions, the Guidelines suggest measuring TSH receptor antibody (TRAb) levels, because TRAb is typically positive in most women with Graves' disease, but negative in the majority of women with postpartum thyroiditis.
To differentiate, doctors will also look for clinical signs of Graves' disease – such as goiter, or thyroid eye symptoms/ ophthalmopathy -- or elevated/normal radioiodine uptake (RAIU) levels to diagnose Graves' disease. The RAIU will typically be low in postpartum thyroiditis.
Radiation can cross over into breast milk, so in women who are breastfeeding, the Guidelines recommend that if an RAIU scan is needed, iodine-123 or technetium be used. These two isotopes have short half-lives – versus the longer half-life of iodine-131 – and a nursing mother can resume breastfeeding several days after the scan.
Treating Graves' Hyperthyroidism in Nursing MothersThe Guidelines recommend that hyperthyroidism be treated, but recommends that if needed, the first choice of medication be the antithyroid drug known as methimazole (brand name Tapazole). Doses up to 20 to 30 mg/d are considered safe for a nursing mother and her baby. The second choice for antithyroid medication after pregnancy is propylthiouracil (known as PTU), at doses up to 300mg/d. Experts have more concerns regarding use of PTU due to problems with liver toxicity that are associated with the drug.
For a nursing mother who is taking antithyroid drugs, the Guidelines recommend that the dosages of antithyroid medication be divided, and taken at times of day that occur after breastfeeding. Breastfeeding infants whose mothers are taking antithyroid drugs should also be screened periodically with thyroid function tests, according to the Guidelines.
Stagnaro-Green, Alex, et. al. "Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum." Thyroid. Volume 21, Number 10, 2011 (Online)