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Too high thyroid dose raises fracture risk in older adults

Older adults prescribed a drug for an underactive thyroid should be regularly monitored to ensure they aren’t taking too high a dose that can put them at an increased risk for hip and other fractures, researchers say.

TORONTO — Older adults prescribed a drug for an underactive thyroid should be regularly monitored to ensure they aren’t taking too high a dose that can put them at an increased risk for hip and other fractures, researchers say.

In a study published online Thursday in the British Medical Journal, a team of Toronto researchers found that older people on medium to high doses of the thyroid medication levothyroxine were 2 1/2 to three times more likely to suffer a fracture than those on a low dose.

The finding applied especially to women, said co-author Dr. Paula Rochon, a geriatrician and vice-president of research at Women’s College Hospital.

People with an underactive thyroid, or hypothyroidism, are usually prescribed levothyroxine, a synthetic form of the hormone thyroxine that’s naturally secreted by the butterfly-shaped thyroid gland located at the base of the throat. The thyroid regulates the body’s metabolism rate, makes proteins and controls sensitivity to other hormones.

Hypothyroidism is common among older people, especially women: it’s estimated that about one in five have the condition and must take medication to normalize hormone levels.

Most hypothyroid patients are diagnosed in early or middle adulthood, but as people age their thyroxine requirements diminish. And although regular monitoring of patients on levothyroxine is recommended, doses often remain unchanged into old age.

This can lead to excess thyroid hormone levels, which can increase the risk of fractures, particularly in older women, Rochon said.

“Thyroid medication is a very important and a good therapy, but what we need to think about is how to give it best to people as they get older,” she said.

“This is a therapy that people may have started when they were quite young and they need to stay on for very long periods of time, indefinitely ... So I think this work highlights the need to continue to carefully monitor the dose of the thyroid medication that’s being given and to decide if there’s a need for any kind of dose adjustment.”

To conduct the study, the researchers looked at health records for 213,511 Ontarians aged 70 to 105, who had at least one levothyroxine prescription dispensed between April 2002 and March 2007. Using hospital data, they found that more than 22,000 patients, or about 10 per cent, had experienced at least one fracture during the study.

Rochon said it’s been known for decades that long-term use of levothyroxine can lead to thinning bone density, which on its own increases the risk of fractures. Older people are also already more prone to falls for a number of reasons, said Rochon.

“Then if you add to that that they’re getting a little bit too much thyroid medication, that may also affect their muscles and make them a bit weaker and also at risk for falls.”

In an accompanying editorial, endocrinology Prof. Graham Leese of Ninewells Hospital and Medical School in Dundee, Scotland, says the ideal thyroxine dose may vary with age and may be unexpectedly low in elderly people.

“It is 120 years since the effect of excess thyroid hormone on bone was first described, yet research in this area still lacks funding,” he writes. “With the prevalence of treated hypothyroidism increasing and the annual economic burden of fractures” in the billions of dollars in the United Kingdom alone, “such research warrants a higher priority.”

The Toronto researchers say their study suggests that the higher fracture risk is dose-related in older adults, and that dosages commonly prescribed by doctors — especially over 0.093 milligrams per day — may be excessive for this population.

“That’s one of our messages — that you need to have this monitored,” said Rochon. “If the dose looks like it’s too high, then it should be decreased and that should be beneficial.”