The second most common drug prescribed for adults is related to bone loss

A popular thyroid medication taken by 23 million Americans may be linked to bone loss, a surprising new study finds.

Levothyroxine – marketed under brand names such as Synthroid – is the second most commonly prescribed drug among older adults in the US. It is consumed by about 7% of the American population.

The drug treats hypothyroidism, also known as an underactive thyroid, by replacing or providing more of a thyroid hormone produced naturally by the body.

The study authors, from Johns Hopkins Medicine, linked levothyroxine use to a greater loss of bone mass and bone density over six years — even in older adults with normal thyroid function.

The preliminary research compared the bone health of 81 levothyroxine users and 364 non-users, who were mostly in their early 70s.

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

For their part, the makers of Synthroid admit that taking too much levothyroxine can lead to increased bone loss – especially in postmenopausal women.

Loss of bone mineral density and bone mass is called osteoporosis, a disease that affects about 10 million Americans age 50 and older.

Bones become weak and more likely to break. With osteoporosis, it can be difficult to move and heal from broken bones. Severe fractures can lead to life-threatening complications.

Hypothyroidism can also lead to serious and potentially fatal complications if left untreated. About 30 million Americans are believed to have the condition, which is often characterized by fatigue, weight gain, and/or hair loss.

The study authors aren’t sure why some patients initially received prescriptions for levothyroxine — and why they still take it.

“The data indicate that a substantial proportion of thyroid hormone prescriptions may be given to older adults without hypothyroidism, raising concerns about subsequent relative thyroid hormone excess even when treatment is targeted at reference range targets,” said Dr. Elena Ghotbi, lead author of the study and a postdoctoral researcher at Johns Hopkins.

The findings will be presented next week at the annual meeting of the Radiological Society of North America.

Meanwhile, adults taking levothyroxine should discuss the drug with their doctor, weighing the benefits of treatment against possible bone loss and other potential negative effects.