A common hyperthyroidism treatment choice in the United States -- radioiodine (RAI) therapy -- is not used at any time in pregnant women, because of the risk of destroying the baby's thyroid gland with radioactive material. This leaves drugs and surgery as the treatment options for women who are hyperthyroid during pregnancy, and drugs are likely where your physician will begin when treating you.
Antithyroid Drugs: A First Choice for Pregnant Women
The key antithyroid drugs -- also known as thionamides -- include propylthiouracil (PTU), methimazole (MMI) and carbimazole. Though effective in pregnant women, babies of mothers taking antithyroid drugs have a high risk of goiter, hypothyroidism or even cretinism. Why are they recommended then? The risk of hyperthyroidism in a mother and fetus itself is greater than the risk of taking a low dose of the medication.
To explain the specifics of an antithyroid drug dosing in a pregnant women, I consulted with UpToDate, the trusted online medical reference resource used by many physicians. According to UpToDate:
"For women with moderate to severe hyperthyroidism complicating pregnancy, we suggest a thionamide as our first choice of treatment. We suggest using PTU rather than methimazole.
"The PTU dose should be adjusted monthly to maintain serum T4 concentrations in the high-normal range and serum TSH concentrations in the low-normal range. We typically use a dose of PTU 50 mg twice daily or less; higher doses (e.g., doses in excess of 200 mg/day) can result in fetal goiter and hypothyroidism.
"To minimize the risk of hypothyroidism in the fetus, we give the lowest dose of thionamide necessary to control thyroid function. In patients with severe hyperthyroidism, full initial doses may be required (PTU 100 mg three times per day) or MMI (10 to -30 mg daily) in order to normalize thyroid function. Our goal is to maintain persistent but minimal mild hyperthyroidism in the mother in an attempt to prevent fetal hypothyroidism.
"It is possible to discontinue the thionamide during the third trimester in one-third of women; the amelioration of hyperthyroidism as pregnancy progresses may be due to a fall in serum TSH receptor-stimulating antibody concentrations and a rise in TSH receptor-blocking antibodies."
Does Each Thionamide Carry Different Levels of Risk?
The antithyroid drug methimazole (sometimes known by the brand name Tapazole) more easily crosses the placenta, meaning that there is a slightly greater risk to an unborn baby of side effects. In addition, rare instances of a condition called aplasia cutis, which causes scalp defects, have been seen in babies born to mothers who took methimazole during pregnancy. These scalp defects have not been seen in babies of mothers who took PTU.
PTU is also the more water-soluble of the antithyroid drugs, and therefore doesn't transfer from mother to baby as efficiently as the other drugs – the reason it’s recommended for use during pregnancy. In areas where PTU is not available, or when a woman is allergic to PTU, methimazole and carbimazole are used during pregnancy, and no particular problems have been associated with careful use of these drugs.
I’m Concerned for My Baby. How Will My Doctor Decide on My Dose?
Again, though these drugs do carry risk in pregnant women, your doctor’s mission is to use them as minimally as possible to control the hyperthyroidism and reduce the risks it poses to you and your baby on its own.
Typically, doctors recommend the smallest possible dose that will control the condition. The goal is to keep your TSH level in the normal range, or borderline hyperthyroid, using as little of the antithyroid drug as possible.
Since all antithyroid drugs do cross the placenta, however, it’s especially important to follow prescription instructions to the letter.
If I’m Taking One of These Drugs, What Do I Need to Do While I’m Pregnant to Make Sure Everything’s OK?
Keeping up with recommended check-ups is extremely important. The goal is to maintain good control of your thyroid levels throughout the pregnancy, so thyroid tests – including Free T4 – should be performed every 2 weeks at the start of treatment. Once levels normalize, testing should continue every 2 to 4 weeks. This regular monitoring is not just to ensure that thyroid levels are stabilized, but to determine if the antithyroid drug dosage needs to be reduced -- or even eliminated entirely. Most doctors will not discontinue antithyroid drug treatment until after the thirty-second week of pregnancy; because before that time, the risk of relapse is high.
In addition to thyroid testing, at healthcare visits, your pulse will be monitored, along with weight gain and thyroid size. Pulse should remain below 100 beats per minute. You should strive to keep your weight gain within the normal ranges for pregnancy, so speak with your doctor about proper nutrition and what types of physical activity are appropriate for your current condition.
Fetal growth and pulse should also be monitored monthly.>> Next: What about surgery, and beta blockers?