Thousands of Brits who take drugs for a common hormone condition could be at risk of bone loss, concerning research suggested today. 

Roughly one in five Britons are affected by an underactive thyroid, caused by problems with the butterfly-shaped gland in the neck which governs functions from digestion to heart rate and mood.

Yet levothyroxine, pills that cost just a few pence a day to combat the condition, could raise the risk of the bone-thinning disease osteoporosis.

Research has long suggested too much thyroid hormone speeds up bone loss heightening the risk of the disease. 

But now, the US scientists who assessed the drug on dozens of patients, found this risk was still raised among those with 'normal' levels of the thyroid-stimulating hormone. 

Professor Shadpour Demehri, an expert in radiology at John Hopkins University in Maryland and study co-author said: 'Our study suggests that even when following current guidelines, levothyroxine use appears to be associated with greater bone loss in older adults.' 

Dr Elena Ghotbi, study lead author and postdoctoral research fellow at John Hopkins University added: 'Data indicates that a significant proportion of thyroid hormone prescriptions may be given to older adults without hypothyroidism.

'This raises concerns about subsequent excess of thyroid hormone even when treatment is targeted to reference range goals.'

Levothyroxine, sold under the brand name Eltroxin, is one of the most widely prescribed drugs in the UK.

Around 1.36 million people in England take it and prescriptions rose by 9 per cent between 2016 and 2022, according to figures published in The Lancet last year. 

In the study, researchers assessed 81 patients on levothyroxine and 364 non-users, with an average age of 73 and thyroid-stimulating hormone level of 2.35.

Normal levels are typically 0.4 to 4.0 milliunits per liter (mU/L). 

Over a follow-up of six years, they found those on levothyroxine suffered a 'greater loss of total body bone mass and bone density'. 

Patients taking the drug who had a 'normal' thyroid-stimulating hormone level also suffered bone mass and density loss, the researchers added. 

Dr Jennifer Mammen, study co-author and associate professor of endocrinology at Johns Hopkins, said adults taking levothyroxine should discuss their treatment with a medic and regularly monitor their thyroid function tests. 

She said: 'A risk-benefit assessment should be conducted, weighing the strength of the indications for treatment against the potential adverse effects of levothyroxine in this population.'

The research will be presented in full at the Radiological Society of North America's annual meeting next week in Chicago. 

According to the British Thyroid Foundation, an underactive thyroid is not a risk factor for osteoporosis. 

However, adults prescribed levothyroxine should have regular blood tests, at least once a year, to ensure thyroid hormone levels are 'not too high', they advise.

'Continuous high thyroid hormone levels may lead to developing or worsening of low bone density and osteoporosis.'

At least one in 20 people in the UK has a disorder of the thyroid, figures show. 

The thyroid produces two key hormones — triiodothyronine (T3) and thyroxine (T4) — which help regulate key body functions, including heart rate, temperature and mood.

Hypothyroidism, where the gland produces too few hormones, leading to weight gain, fatigue, feeling the cold, constipation and dry skin and hair.

Women are ten times more likely to be affected than men, although it's not clear why — and the most common trigger for both thyroid problems is an auto-immune condition, where the body mistakenly attacks the thyroid tissues.

There are 3.5 million people living with osteoporosis in the UK, but it is often known as a 'silent disease' as there are no symptoms until someone breaks a bone.

Half of all women and one in five men over 50 will suffer a fracture, most often in the spine or hips, because of the disease, which causes bones to become so fragile a break can be triggered by a cough or sneeze. 

Such fractures are the fourth biggest cause of disability and premature death in the UK.