Hypothyroidism: up levothyroxine dose after pregnancy confirmed

OB/GYN News, June 1, 2004 by Michele G. Sullivan

ALEXANDRIA, VA. -- Women of reproductive age who have clinical hypothyroidism should be counseled to increase their levothyroxine by two doses per week as soon as pregnancy is confirmed and to have their thyroid-stimulating hormone levels checked as soon as possible, Dr. P. Reed Larsen said at a symposium sponsored by the American Thyroid Association.

The two additional doses equal an increase of about 30% in levothyroxine (LT4) intake--enough to offset the increased thyroxine need that has been shown to arise in half of pregnant women by the eighth gestational week, said Dr. Larsen of Harvard Medical School, Boston.

A growing body of evidence suggests that maternal hypothyroidism in the first trimester puts the fetus at increased risk for neurodevelopmental abnormalities, including a decreased IQ.

Preconceptional counseling is necessary because most women, even if they regularly see an endocrinologist for their thyroid condition, don't seek specific counsel on the subject when they conceive, Dr. Larsen said. "Everyone's excited about a new pregnancy, and calling the endocrinologist about their thyroid hormone dose isn't tops on their list of things to do."

Even a 30% increase in LT4 may not be enough to optimize the thyroxine levels of pregnant women, depending on the etiology of their hypothyroidism. Women whose thyroid has been ablated because of Graves' disease or radiation therapy will probably need even more of an increase. It's important to test repeatedly--every 4 weeks--and to adjust the dosage until the thyroid-stimulating hormone levels are 0.5-3.0 mIU/L.

The full increase in LT4 can be taken immediately without fear of inducing hyperthyroidism, Dr. Larsen noted. "Thyroxine has a half-life of 5-6 days, so if you make an abrupt change in the dose, in most patients it takes 4-5 weeks to equilibrate to a new value."

It's important to remember that the increased need for thyroxine stops the day the patient delivers. "Reduce the dosage to the prepregnancy level and recheck the serum levels 4-6 weeks after the baby is born," he said at the meeting, which was also sponsored by the American Association of Clinical Endocrinologists.

Maintaining the increased LT4 dosage after delivery could result in a suppression of thyroid-stimulating hormone. "This is further evidence that this increased need for thyroxine is a transient phenomenon," Dr. Larsen noted.

BY MICHELE G. SULLIVAN

Mid-Atlantic Bureau