January is Thyroid Awareness Month, and thyroid medication is among the most prescribed in the U.S. Levothyroxine, a common thyroid supplement, is currently the second most filled drug at American pharmacies, according to data from GoodRx.
So why is thyroid medication so popular?
Dr. Elizabeth Pearce, an endocrinologist and president of the American Thyroid Association, says thyroid disorders disproportionately impact women, especially as they age.
"The vast majority of thyroid disease is in women," she points out. "Hypothyroidism, which is 7 to 8 times more frequent in women than in men."
Hypothyroidism, the most common thyroid disorder, is when the thyroid produces too little hormones.
According to Pearce, hypothyroidism affects 1 percent of the population enough to need thyroid supplements and 5 to 10 percent at a subclinical level, many of whom don't need medication.
A blood test measuring TSH levels determines whether you have a thyroid disorder.
Pearce says half of 1 percent of Americans suffer from hyperthyroidism, where the thyroid gland produces too many hormones.
She notes that thyroid cancer, which is usually treatable, is on the rise.
"About 54,000 adults diagnosed with thyroid cancer in the U.S. in 2018, which means about 1 percent of individuals in the U.S. will be diagnosed with thyroid cancer in their lifetimes, based on current numbers," Pearce points out.
She says the reasons for thyroid disease in the U.S. are not well understood. One cause around the world is iodine deficiency, which hasn't been a problem in the U.S. for about a century.
But Pearce says one important group has become more iodine deficient here in the last 10 years: pregnant women.
"Specifically for women who are planning pregnancy, or who are pregnant, or who are breastfeeding, the current recommendations are that those women should be taking a daily supplement containing 150 micrograms of iodine to make sure they're getting what they need," she states.
Some prenatal vitamins have iodine, but some don't. Pearce also suggests taking this supplement because iodine deficiency is associated with lower IQ in fetal development.