Side Effects of Synthroid, Similar Thyroid Drugs May Increase Risk of Bone Loss: Study

Synthroid and other widely used thyroid drugs, which patients often take for the duration of their lives after a hypothyroidism diagnosis, may increase a person’s risk of losing bone density, according to the findings of a new study.

Known by the generic name levothyroxine, Synthroid is a synthetic hormone used to replace thyroxine, also known as T4. The drugs are widely used for the treatment of hypothyroidism, which occurs when the thyroid gland does not produce enough thyroxine hormone on its own.

The condition can cause fatigue, weight gain, hair loss and other symptoms, and if left untreated can lead to serious and potentially fatal complications. Synthroid is the second most commonly used prescription medication in the U.S., with more than 23 million Americans, or 7% of the population, taking the drug daily.

However, in a new study being presented this week at a conference of the Radiological Society of North America (RSNA), researchers from the Russell H. Morgan Department of Radiology and Radiological Science and the Endocrinology Department at Johns Hopkins warn that Synthroid side effects may be associated with bone loss, potentially increasing the risk of fractures.

The researchers, led by Dr. Elana Ghotb, used data from the Baltimore Longitudinal Study of Aging and included information from 81 adults who were 65 years and older. Participants were taking Synthroid, had at least two study visits, and thyroid function tests had to be consistently within thyroid stimulating hormone (TSH) reference ranges of 0.4 to 5.0 micro-units per milliliter.

The participants were compared to 364 Synthroid non-users with a TSH level of 2.35 at the initial visit. Researchers also measured changes in bone density and bone mass over the course of the study.

According to the findings, the researchers determined that taking Synthroid was linked with a higher loss of total body bone mass and bone density. Even participants who had TSH levels within the normal range showed signs of loss of bone mass and bone density.

People begin to lose bone density as they age, but losing a greater amount of bone mass from taking medication puts older people at risk of suffering bone fractures, which can lead to serious long-term complications.

The study data indicates the decrease in bone density is consistent even after taking into account other risk factors like weight, smoking, other medication use and alcohol use.

Researchers said taking Synthroid is linked to bone density loss even when current thyroid treatment guidelines were being followed.

“Data indicates that a significant proportion of thyroid hormone prescriptions may be given to older adults without hypothyroidism, raising concerns about subsequent relative excess of thyroid hormone even when treatment is targeted to reference range goals,” Ghotbi said in a press release. She urged adults taking Synthroid to talk to their doctor about their treatment and regularly monitor their thyroid function tests.

The findings are considered preliminary until published in a peer-reviewed journal.