Researchers are raising concerns about levothyroxine, the second most commonly prescribed thyroid medication for older adults in the U.S., as new findings link its use to bone loss.

Levothyroxine or L-thyroxine is a synthetic version of the hormone thyroxine commonly given to patients with underactive thyroid. People with underactive thyroid or hypothyroidism show signs such as fatigue, increased heart rate, depression, dry skin and hair, muscle cramps, constipation, weight gain, and memory issues.

Around 23 million U.S. adults take levothyroxine, which is sold under the brand names Synthroid, Levoxyl, Unithyroid, and Tirosint. However, researchers of the latest study noted that many of them continue the medication for years without checking if it's still required or fully understanding the reasons for its initial prescription.

"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," said the study's lead author Dr. Elena Ghotbi, postdoctoral research fellow at Johns Hopkins University School of Medicine in Baltimore, Maryland.

Studies have shown that improper dosage of levothyroxine, both excessive or insufficient quantities may affect growth and development, cardiovascular function, bone metabolism, reproductive function, cognitive function, emotional state, gastrointestinal function, and glucose and lipid metabolism.

The latest study investigated whether levothyroxine use in older euthyroid adults (those with normal thyroid function), but higher thyroid hormone levels within the normal reference range, is linked to increased bone loss over time.

For the study, the researchers evaluated bone loss over time of 81 euthyroid levothyroxine users and compared it against 364 non-users who had similar risk factors such as age, gender, height, weight, race, medications, smoking history, and alcohol use.

The researchers noted that even in participants whose TSH levels were within the normal range, levothyroxine use was associated with greater loss of total body bone mass and bone density after a follow-up of around 6.3 years.

"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-senior author and professor of radiology at Johns Hopkins.

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