Tiredness can be a sign of a thyroid problem
People with suspected thyroid disorders are being mistreated and misinformed, experts have warned.
British Thyroid Association doctors say some people are being given the wrong tests and the wrong treatment.
NHS doctors abide by expert guidelines - but the BTA says the problem comes when patients go outside the NHS.
Around 3% of the UK population has an underactive thyroid, which should be diagnosed with a blood test and treated with a synthetic hormone.
An under-active thyroid, or hypothyroidism, develops when the thyroid gland produces too little thyroxine, and it is becoming more prevalent because of the ageing population.
Symptoms can include being very tired, feeling the cold, having difficulties with memory or concentration, weight gain and fertility problems.
These are symptoms that can mimic other conditions, and experts warn an incorrect diagnosis could mean some patients could suffer harmful effects from excess thyroid hormones, while other serious conditions may go undiagnosed.
The Royal College of Physicians (RCP) recently set out guidance for how hypothyroidism should be diagnosed and treated in the UK.
It says the only accurate way to diagnose a thyroid disorder is via a blood test which measures hormone levels, and the only scientifically proven way of treating the condition is by topping up a patient's natural thyroxine levels with a synthetic form of the hormone.
But the BTA warns that information on the web and in the media about alternative ways of diagnosing and treating the condition are leading people to turn to alternative methods of diagnosis and treatments.
It says urine tests, saliva tests and measuring body temperature are not reliable ways of diagnosing the condition.
Dr Amit Allahabadia, the secretary of the BTA who wrote the editorial, said: "This is potentially an enormous problem, given that in any one year, one in four people in the United Kingdom have their thyroid function checked.
He added: "I think it is essentially doctors who are outside the NHS [who] may be misdiagnosing the condition.
"Patients may go to see them when they think they have an under-active thyroid, or when tests have shown they have normal hormone levels but they still feel ill."
Dr Allahabadia said he believed a "significant minority" of patients were affected, either directly through misdiagnosis or mistreatment or because they were being confused by inaccurate information.
Professor Peter Trainer, who chairs the clinical committee of the Society for Endocrinology which represents the specialists who treat thyroid disorders, said: "Our sympathy has to lie with the patient because there is potentially misleading information available on the web.
"It can be confusing for patients, and it can be difficult for GPs when they are confronted with that information, which is why the RCP guidance was published."