New Study Links Popular Thyroid Drug to Increased Bone Loss

The common hypothyroidism medication levothyroxine is linked to bone loss in older adults, according to a study presented at the Radiological Society of North America annual meeting on Dec. 1.

Levothyroxine, marketed under brand names including Synthroid, is a synthetic version of thyroxine, a hormone produced by the thyroid gland. It’s the second most commonly prescribed drug among older adults in the United States, with about 23 million—or 7% of—Americans using it daily. 

Doctors prescribe levothyroxine to treat hypothyroidism, which occurs when the thyroid gland produces too little thyroxine. Without treatment, weight gain, fatigue, hair loss, and more serious complications can occur. You can also have too much thyroxine, which has been linked with bone fractures. 

But the new study suggests that even in adults whose thyroid levels have stayed within the accepted normal range, taking levothyroxine daily over six years is associated with greater bone loss.

“Our study suggests that even when following current guidelines, levothyroxine use appears to be associated with greater bone loss in older adults,” said Shadpour Demehri, MD, co-senior author and professor of radiology at Johns Hopkins, in a statement. 

The study included participants aged 65 or older with hypothyroidism, 81 of whom were taking levothyroxine daily, and 364 non-users. 

Participants had at least two doctor visits, during which blood work confirmed that they had normal levels of both thyroid-stimulating hormone (TSH), which measures the hormone in the brain that regulates thyroid function, and free T4, which reflects levels of thyroxine. They also had scans to measure bone density and mass. 

Researchers found that after about six years, those on levothyroxine lost more total bone mass and total body bone density than people not taking it. The association was stronger as thyroxine levels rose.

“When thyroid medication is given in excess, then people have more thyroid hormone than they need,” co-senior author Elena Ghotbi, MD, a postdoctoral research fellow at Johns Hopkins University School of Medicine in Baltimore, Maryland, told Health. Having a relative excess of thyroid hormone can increase bone resorption and lead to a breakdown of bone tissue, resulting in the bone disease osteoporosis. 

Though the study showed a link between bone loss and levothyroxine, Ghotbi stressed that it didn’t prove causation and that there are other possible explanations for the bone loss. For example, someone more likely to lose bone may also be more likely to take the medication.

Generally speaking, there isn’t strong evidence that levothyroxine use among euthyroid patients, or those with a normal TSH and free T4 levels, causes bone loss, Marilyn Tan, MD, a clinical associate professor in endocrinology and internal medicine at Stanford University who reviewed the preliminary research, told Health.

Aaron Schulman, MD, an assistant professor of endocrinology at Weill Cornell Medical College who was not on the research team, told Health that the trial was well designed. However, he said he would have liked to see participants’ TSH levels and know why patients were put on levothyroxine to begin with.

Schulman doesn’t advise going off levothyroxine in light of the study’s results. “Many patients absolutely need to take levothyroxine,” he said. “Those who have had their thyroid gland surgically removed, for example, would become extremely ill if they stopped taking their levothyroxine.” 

Levothyroxine is still the gold standard in hypothyroidism treatment, Schulman pointed out. “Levothyroxine remains the safest and most widely used option to treat underactive thyroid,” he said. He added that other options may come with an even higher risk of bone loss, especially in older adults.

Doctors are less concerned about bone loss in premenopausal women who take levothyroxine and have normal thyroid hormone levels, Schulman said. That’s because people who’ve gone through menopause may have more bone loss to begin with due to lower estrogen levels. 

But if you’re concerned about taking levothyroxine, experts suggest speaking to your doctor before making any changes.

Because some evidence shows that not everyone on levothyroxine has hypothyroidism—sometimes people with normal hormone levels take it for hypothyroidism symptoms—patients may want to review their diagnosis and treatment goals with their provider, Ghotbi said.

“This study suggests that there is a continuum of bone risk that extends within and outside the normal reference range,” Schulman said. “If that is the case, the answer may not necessarily be stopping the levothyroxine, but rather, adjusting the dose to maintain thyroid hormone levels targeting a specific part of the normal reference range.”