Hyperthyroidism in pregnancy

Poorly controlled hyperthyroidism during a pregnancy is associated with poor outcomes. 

Conditions associated with uncontrolled overt hyperthyroidism during pregnancy:

  • Spontaneous abortion
  • Premature labor
  • Low birth weight
  • Stillbirth
  • Preeclampsia
  • Heart failure
  • Very rare cases of thyroid storm caused by labor, infection, preeclampsia, or cesarean section
The causes of hyperthyroidism during pregnancy are no different from the usual causes.  Graves hyperthyroidism is the most common cause. Paradoxically Graves disease usually becomes less severe during the later stages of pregnancy. Generally an endocrinologist is the type of physician who is primarily managing hyperthyroidism, especially during a pregnancy.

The diagnosis of hyperthyroidism during pregnancy may be difficult 

The blood tests used to diagnose hyperthyroidism are more difficult to interpret due to the normal pregnancy changes in thyroid function. TBG excess causes high serum total T4 concentrations, but not high serum free T4 concentrations. High serum hCG concentrations during early pregnancy can result in hCG-mediated hyperthyroidism.  The diagnosis of hyperthyroidism in pregnant women should be based primarily on a serum TSH value <0.01 mU/L and also a high serum free T4 value. Since most lab companies do no provide pregnancy reference ranges for free T4 or free T4, the best test is the Free Thyroxine Index (T7) which remains constant weather pregnant or not. This test is a ratio of the total T4 and the T4 uptake. Free T3 measurements may be useful in women with suppressed serum TSH concentrations and normal or minimally elevated free T4 values. Because radioactive iodine is not usable during pregnancy, the exact causes of hyperthyroidism may not be found during a pregnancy. 

Treatment options for pregnant women with hyperthyroidism are limited 

The treatment options are limited to thionamide pills (antithyroid medications like PTU or methimazole) or late trimester surgery. The goal of treatment is to maintain the mother's serum Free T4 Index in the upper 1/4 of the normal range using the lowest dose of antithyroid medication. This requires continual assessment of free T4 index every 4-6 weeks with appropriate adjustment of medication.  One to 5 percent of neonates born to women with Graves disease have hyperthyroidism due to transplacental transfer of TSH receptor-stimulating antibodies. The incidence is unrelated to maternal thyroid function. The complications of neonatal hyperthyroidism (High fetal heart rate, fetal goiter, advanced bone age, poor growth, craniosynostosis, cardiac failure. and hydrops) are managed by high risk obstetricians and the pediatricians. The endocrinologists at Houston Thyroid and Endocrine specialists will help you manage your hyperthyroidism during pregnancy. 

Related pages: Thyroid in Pregnancy HCG mediated hyperthyroidism Hyperthyroidism in pregnancy Hypothyroidism in Pregnancy Post partum thyroiditis