Testing and treatment for underactive thyroid is increasing in Australia. Why?

<span>An underactive thyroid, or hypothyroidism, is thought to affect about 4% of the population, though it’s more prevalent among women and those over 40.</span><span>Photograph: Bsip Sa/Alamy</span>
An underactive thyroid, or hypothyroidism, is thought to affect about 4% of the population, though it’s more prevalent among women and those over 40.Photograph: Bsip Sa/Alamy

“Sends shivers down my spine, body’s aching all the time” – Freddie Mercury (probably) wasn’t singing about thyroid dysfunction, but the lyrics fit the bill.

As with the other symptoms of an underactive thyroid – including lethargy, brain fog, weight gain, and low mood – feeling cold and having aching muscles and joints are unpleasant and can affect a person’s quality of life, but have myriad possible causes. So when patients present to their GP complaining of these symptoms, they and their doctors are led into a careful dance of trying to deduce the clinical origin.

According to publicly available data, a growing number are landing on an underactive thyroid. Testing and treatment for hypothyroidism in Australia is on the rise. If you spend much time speaking with women around middle-age or older, this might not come as a surprise.

In 2014, 7,367,907 thyroid function tests were covered by Medicare, but by 2023 that number hit 9,893,443. In the same decade, prescriptions for levothyroxine (the synthetic form of thyroid hormone, mostly prescribed for an underactive thyroid), grew from 1,596,232 to 2,095,454.

According to an analysis by the chief of the medical advisory committee of the Endocrine Society of Australia, Assoc Prof Shane Hamblin, these increases outstrip population growth (though this doesn’t represent the dosages prescribed, which may also vary).

Experts however, say this might not be entirely due to an actual increase in the disease in the population – and that while not detecting or treating these conditions can have significant effects on patients, thyroid testing and treatment should not be undertaken lightly.

What is the thyroid?

The thyroid is a bow tie-shaped gland wrapped around the front of the windpipe that produces two hormones (T3 and T4) with crucial roles across almost every part of the body, from metabolism to heart function.

If production of these hormones dips, the pituitary gland “cracks the whip” by releasing more thyroid-stimulating hormone (TSH), prompting the thyroid to increase its output.

An underactive thyroid, or hypothyroidism, is thought to affect about 4% of the population, though it’s more prevalent among women and those over 40.

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Globally, iodine deficiency is a major cause, but in Australia the autoimmune disease Hashimoto’s thyroiditis is the biggest culprit. Other causes include surgery for goitres or cancers, along with certain medications, such as lithium. Too much iodine, through supplements or diet, can also contribute.

In mid-2022, the health department wrote to more than 5,000 GPs who had higher than average thyroid testing rates to remind them of the clinical conditions for requesting thyroid function, iron, and other tests – a move criticised by some as targeting tests disproportionately ordered for women.

Testing asymptomatic people’s thyroid function is not recommended in the national guidelines for GPs. This is because treating people who have abnormal blood test results but no symptoms may consume resources with little to no benefit to the patient – or put them at risk of other illness, contributing to overdiagnosis and overtreatment.

Why the rise?

Hamblin speculates that rather than a true increase in the prevalence of hypothyroidism, the testing and prescription figures indicate more doctors are including thyroid function within broader testing – therefore detecting more cases, including those classed as subclinical (where TSH is elevated but thyroid hormone is still normal).

He suspects some mild hypothyroidism cases that would previously have been monitored are now being prescribed medication. But he stresses that’s not necessarily inappropriate – provided other causes have been ruled out, and the response to the medication is monitored. “It’s not necessarily saying you would be wrong to intervene, just that the evidence isn’t strong that it makes much difference if you intervene early,” he says.

The RACGP guide for subclinical hypothyroidism recommends not prescribing medication in the first instance, but retesting after two to three months, since many patients who are simply monitored won’t end up worsening.

But the guide indicates doctors can trial medication if the patient has symptoms, certain risk factors, or further testing reveals thyroid antibodies (which might indicate Hashimoto’s).

Part of the challenge stems from the fact that the symptoms of underactive thyroid are not specific and are common reasons people see their GPs, Hamblin notes.

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The chair of the RACGP expert committee for quality care, Dr Mark Morgan, says in addition to detecting some hypothyroidism early, “some of the increase might be happening because we have very sensitive tests for thyroid function and minor temporary upsets lead to lifelong treatments with low-dose thyroid supplements”.

Dr Michael Tam, another member of the guide’s steering group, suspects population changes may play a role: as people move into middle and older age, they’re more likely to experience the symptoms that could indicate thyroid dysfunction.

It’s not known why the immune system starts attacking the thyroid, nor why Hashimoto’s affects mainly women.

“There’s also a lot of research into why we seem to be seeing an increase in autoimmune disease of all kinds [in western countries], but again, not specifically linked to the thyroid,” Hamblin says.

Prof Creswell Eastman AO, a world-renowned endocrinologist and principal medical adviser for the Australian Thyroid Foundation, says there’s no good evidence that viral or bacterial infections trigger Hashimoto’s and Graves’ disease (the autoimmune condition involving an overactive thyroid).

“By contrast, a number of viruses, especially mumps, are clearly implicated in or causing subacute thyroiditis, which is a self-limiting disease,” Eastman says.

Eastman says while there have been recent reports of Covid-19 patients developing thyroid dysfunction, most had normal thyroid function three months later.

Where people had an underlying autoimmune thyroid disease, it’s possible – but difficult to prove – that Covid-19 infection could be triggering the onset, Eastman says.

Overintervention and undertreatment: a balance

Severe hypothyroidism, while now rare in Australia (Hamblin estimates they might see two cases a year at a large Melbourne hospital), can be fatal. “Severe hypothyroidism is extremely dangerous,” he says.

He adds: “A lot of people will get moderate hypothyroidism and don’t die, but their quality of life is terrible – and they might go for several years before the penny drops.”

However, experts speaking to Guardian Australia say that any overtreatment for subclinical conditions may have financial or health implications.

“The harms associated with treatment with thyroxine for subclinical hypothyroidism include increasing the risk of atrial fibrillation – a condition where the heart beats erratically and can cause a stroke,” Morgan, says. “Other risks include osteoporosis – reducing the strength of your bones, and fractures.”

However, both atrial fibrillation and osteoporosis are common conditions – becoming more common as people age – and it’s unknown how much the treatment of subclinical hypothyroidism contributes to the numbers, he says.

“There are also costs involved in taking and monitoring lifelong medication, so these medications should only be used when there is clear evidence of benefit over harms.”

And in busy, modern life, there can be many causes for a person experiencing the symptoms associated with hypothyroidism, Tam says. And “there are lots of reasons why thyroid function tests can be a little bit outside the range”.

Subclinical results may distract from finding the true or complex causes of a patient’s symptoms – and better targets for treatment, Tam says.

He says a diagnosis should not be made lightly as there’s also the risk of changing how people think about their health.

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You may have only had one slightly abnormal result, Tam says, “but if you think you have a disease, you’ve taken on the label of someone living with a chronic illness”. This can lead to people framing more of their experiences – and other symptoms – around the idea they’re chronically ill, he says.

What about pregnancy?

As with the GP guide, the national guide for obstetricians doesn’t recommend treating subclinical hypothyroidism in pregnancy, citing poor evidence of benefit.

However, the question of treating subclinical hypothyroidism during pregnancy is an ongoing area of research and debate among professional bodies, Eastman says.

“We don’t have all the answers but why would you not treat and not provide benefit when the treatment is safe, simple and unlikely to cause side effects,” he says.

The evidence is clear that overt hypothyroidism however (where both the TSH levels and the thyroid hormone levels are abnormal) can have severe consequences in pregnancy and breastfeeding in particular, and can impact life quality in non-pregnant people if left undiagnosed and untreated.

The role of lifestyle

There are no proven alternatives to medication, Hamblin adds – unless someone were on a very restricted diet or had another reason for severe deficiencies, diets aiming to improve thyroid dysfunction would be unnecessary in Australia. Eastman, however, is concerned that many Australians generally have an iodine deficiency, though he can’t speculate on its contribution to the current prevalence of hypothyroidism.

While it can’t repair thyroid function, Hamblin recommends managing stress.

“How do you prevent an autoimmune condition from getting worse? The advice we give is: try not to burn the candle at both ends. Make sure you get enough sleep, good nutrition, and exercise, because we know stress can make autoimmune conditions worse.”