Hyperthyroidism and pregnancy


  • Hyperthyroid women may not conceive easily because of menstrual irregularities associated with hyperthyroid state.
  • In pregnant state hyperthyroidism is usually overlooked because of similar features such as tachycardia (increased heart rate), warm skin, heat intolerance, increased sweating and palpable thyroid.
  • Moreover a hyperthyroid pregnant state is associated with a greater risk for miscarriage, spontaneous abortion, fetal growth retardation, premature delivery, and may be pre-eclampsia. Fetal death may occur as a result of chromosomal abnormalities such as Down’s syndrome. Hence it is important that hyperthyroidism be detected earlier and treated appropriately during pregnancy.
  • Hyperthyroidism (over functioning of thyroid) in pregnancy is commonly due to Grave’s disease.


  • If a pregnant women experiences a rapid heart rate (> 100 beats /min.) and weight loss, she should be investigated for hyperthyroidism.
  • Laboratory investigations are helpful in confirming the diagnosis:
    • Free T4 is elevated and TSH is suppressed. Although T4 is elevated in most pregnant women, values over 20µg/dl are encountered in hyperthyroidism.
    • The T3 resin uptake which is low in normal pregnancy (because of high concentration of Thyroxine Binding Globulin-TBG) is normal or high in hyperthyroidism.
    • The level of thyroid stimulating immunoglobulin may be elevated especially if hyperthyroidism is because of Grave’s disease. It is essential to check the levels of thyroid-stimulating immunoglobulin with blood tests every few months if a women is pregnant because elevated levels may have adverse effect on the newborn baby.
    • Radioactive iodine scanning is not recommended in pregnant women as fetal thyroid can be destroyed by the radioactive material, resulting in an underactive thyroid gland, leading to physical and mental retardation.


In order to prevent any complications during pregnancy it is essential to detect hyperthyroidism early and treat it appropriately.

  • The drug of choice is Propylthiouracil in the smallest dose possible. However it is important to remember that anti-thyroid medications can be harmful to the fetal thyroid and the fetus may be born hypothyroid (i.e. not enough thyroid hormones to sustain normal development). Only minimal amounts of propylthiouracil are transferred to the mother’s milk, so breast feeding is safe. 
  • Thyroidectomy is reserved for women who are allergic or resistant to anti-thyroid drugs or those who have a very large goiter.


Untreated hyperthyroidism may affect the mother as well as the developing fetus.

Complications in mother include

  • Heart disease especially heart failure
  • Pre-eclampsia or eclampsia
  • Premature delivery
  • Thyroid storm

Complications in the baby include

  • Transient thyrotoxicosis due to transfer of Thyroid stimulating hormone antibodies from the mothers blood to the fetus. Even with proper medication the TSH antibodies may remain in mother’s blood and may be passed to the fetus. Hence it is essential to test the baby immediately after birth for possible overactive thyroid gland.
  • Intrauterine growth retardation
  • The antithyroid drugs like propylthiouracil or methimazole are transferred to the mother’s milk (usually PTU is preferred as it passes into the breast milk less readily than methimazole) and higher doses of these medications can block thyroid activity in the newborn baby causing hypothyroidism. This may result in intellectual and growth retardation. Hence only those mothers who are on small doses of PTU should be allowed to breast feed. The children of such mother should be followed-up by the pediatrician.