Patients on with an underactive thyroid taking medication require smaller amounts as they get older, if treatment levels are not adjusted their excessive doses considerably raise the likelihood of bone fractures, Canadian researchers revealed in the BMJ (British Medical Journal). The authors stress that patients need regular dose monitoring to check whether their treatments need altering.

Levothyroxine, a synthetic form of the thyroid hormone, thyroxine, is commonly prescribed for hypothyroidism (underactive thyroid gland).

The majority of patients with an underactive thyroid are diagnosed before they are old. When we age we need less thyroxine. In many cases patients’ doses are not being reduced when they enter old age, leading to a higher risk of hyperthyroidism (thyroid hormone levels are too high). Hyperthyroidism raises the risk of fracture, especially among older females.

The authors explain that prior studies that looked at a possible link between levothyroxine and bone fractures have produced conflitcting results. Lorraine L Lipscombe, from the Women’s College Research Institute, Canada, and team decided to find out what effect levothyroxine doses might have on the risk of fractures among elderly patients.

They gathered data from a population-based study from Ontario involving 213,511 individuals aged at least 70 years – they had all been prescribed levothyroxine at least once between 1 April 2002 and 31 March 2007. They identified fractures from hospital records – each case was matched to five individuals who had not had a fracture yet (controls).

They identified both the cases and controls as current users, recent past users (had stopped the medication 15 to 180 days before the study began), or remote users (had stopped the medication at least 180 days before the study began) of levothyroxine.

During the study period 10.4% (22,236) of patients had at least one fracture.

They found that the risk of a fracture was considerably higher among the current and past levothyroxine use patients, compared to the remote use ones. Current users on medium and high levothyroxine doses had a much higher fracture risk than those on low doses, the authors added.

There was a clear link between hip and other bone fractures and higher levels of dosage among males and females, even after other fracture risk factors were taken into account.

The researchers concluded:

“Our findings provide evidence that levothyroxine treatment may increase the risk of fragility fractures in older people even at conventional dosages, suggesting that closer monitoring and modification of treatment targets may be warranted in this vulnerable population.”
Accompanying Editorial
Professor Graham Leese at Ninewells Hospital in Dundee warns thyroxine requirements may be much lower than expected among elderly patients.

Even though we have known about the risk of bone fractures linked to excessive thyroid hormone, the amount of funding in this area continues to be too low.

He wrote:

“With the prevalence of treated hypothyroidism increasing, and the annual economic burden of fractures in the United Kingdom currently estimated at €5.8bn (ВЈ5.1bn; $8.4bn), such research warrants a higher priority.”

“Levothyroxine dose and risk of fractures in older adults: nested case-control study”
Marci R Turner, medical resident, Ximena Camacho, analyst, Hadas D Fischer, epidemiologist, Peter C Austin, senior scientist, Geoff M Anderson, professor, Paula A Rochon, Lorraine L Lipscombe
BMJ 2011; 342:d2238