The most prescribed drug in Canada is a pill for underactive thyroid. Now, an international panel of doctors says too many people are taking it unnecessarily for what is a mild abnormality.

The strong recommendation against treating "subclinical hypothyroidism" focuses on levothyroxine. The hormone is sold as Synthroid — the top dispensed medication in Canada in 2017 — and also under the brand name Eltroxin.

In full-fledged hypothyroidism, the drug is used to replace a natural hormone called thyroxine, which plays an important role in heart and digestive function, metabolism, brain development and bone health.

The subclinical form is really just a lab abnormality, not an illness, hormone specialists say.

In recommendations for subclinical hypothyroidism published in this week's BMJ , Dr. Thomas Agoritsas, an assistant professor of medicine at Hamilton's McMaster University, and other panellists concluded levothyroxine hormones "consistently demonstrate no clinically relevant benefits for quality of life or thyroid-related symptoms, including depressive symptoms, fatigue and body mass index."

"From what we know, it's very likely [levothyroxine treatment] won't work," said Agoritsas, who also works at Geneva University Hospitals.

The researchers reviewed data from 21 clinical trials involving more than 2,100 participants, many over the age of 65. They compared the effects of thyroid hormone treatment with no treatment or placebo among adults with subclinical hypothyroidism.

Synthroid was the top prescribed drug in Canada in 2017, IQVIA reports. The drug, a hormone called levothyroxine, is also sold under the brand name Eltroxin. (Craig Chivers/CBC)

The panel's decision was based on the absence of benefits, uncertainty about potential harms and the burden of taking a daily medication and getting frequent, ongoing blood tests to monitor the dosage, Agoritsas said.

Normal thyroid hormone levels

The pituitary gland in our brains produces thyroid-stimulating hormone (TSH), encouraging the thyroid gland in the neck to produce thyroxine, also called T4. If a blood test shows elevated TSH, doctors assume there isn't enough T4, and more TSH is being produced to stimulate more T4 production. But the same blood test can also show normal levels of T4 in subclinical cases.

"The brain probably detects that the thyroid gland is mildly dysfunctional and compensates by stimulating it more," Agoritsas explained. The brain "does it successfully, because the hormone levels are normal in the blood." That means, the fact that the T4 level is normal shows the extra TSH production is already working and doesn't need to be treated with levothyroxine.

People with subclinical hypothyroidism have high levels of thyroid stimulating hormone (indicated as TSH above the normal line) but normal levels of thyroxine (indicated as T4 on the normal line). Studies show these patients don't benefit from the drug levothyroxine. (Dr. Thomas Agoritsas/CBC)

Nonetheless, there's an increasing tendency to treat subclinical hypothyroidism in many countries, including Canada, said Dr. Deric Morrison, an endocrinologist, or hormone specialist, at St. Joseph's Hospital in London, Ont.

Morrison speculated on the reasons why. "People aren't feeling well, and thyroid functions are widely available and easy to check and the symptoms are non-specific. If something comes back abnormal and somebody's not feeling well then I think intuitively it makes sense to try treating that abnormality and see if it makes someone better.

"But the evidence is now suggesting for most of those people, treating that number doesn't make them better."

5% of population

About five per cent of the general population has subclinical hypothyroidism, which rises to 10 to 15 per cent among those age 65 and older, the panel said.

Guidelines for physicians tend to recommend prescribing hormones for those with TSH levels above 10 mIU/L (milli-international units per litre). A normal TSH level is 0.4 to 4.0 mIU/L.

But another expert cautions against rushing to the medicine cabinet to throw out the levothyroxine pills.

Thyroid hormones are powerful drugs, and patients should not stop taking their medication without consulting a physician, Prof. Helen Stokes-Lampard, chair of the Royal College of General Practitioners, said in a statement in response to the BMJ article.

'Crushing' fatigue

Melissa Armster, 33, of Waterloo, Ont., was diagnosed with a form of hypothyroidism that's now being treated. She recalled feeling extreme fatigue two years ago.

"It was really crushing, because I remember being at work and one day I was so tired I actually hid under a co-worker's desk and slept there for 20 minutes just so I could function for the remainder of the day," she said after an information session this week in Kitchener, Ont., hosted by the Thyroid Foundation of Canada. She said she felt better within days of starting treatment.

There are different schools of thought about treatment, said Cassandra Howarth, president of the local chapter of the foundation.

"I have seen people who have been treated and they have indicated that they felt better, and other people that doctors have advised that they are close but not quite at the range where they would be treated," Howarth said. "They are still here looking for answers."

When a patient has symptoms such as fatigue, dry skin and weight gain, Agoritsas will check hormone levels with a blood test. If no abnormalities are found, he'll discuss other ways to cope. But if, in discussions about treatment, a patient wants to try a short-term trial of levothyroxine, he will prescribe it, without expecting to have the patient stay on it for life.

The new recommendations don't apply to women trying to become pregnant or patients with very high TSH levels above 20 mIU/L. It may also not apply to patients with severe symptoms or those under age 30.