Hundreds of thousands of people in the UK are being prescribed thyroid drugs unnecessarily, according to international researchers.
They advise against lifelong hormone treatment for mild underactive thyroid problems (hypothyroidism), saying there is not enough proof it helps.
The daily tablets do not appear to ease symptoms such as tiredness, low mood and weight gain, they claim in the BMJ.
Experts stressed that patients should not stop taking their medication.
If they have questions, they should discuss them with their GP at their next routine medication review.
Hypothyroidism affects about one in 20 people but is more common in older age and among women.
The thyroid is a gland in the neck that makes hormones to help control energy levels and growth.
Patients with hypothyroidism are often prescribed lifelong daily pills - levothyroxine (T4) or a more expensive drug, liothyronine (T3) - to replace the missing thyroid hormone.
More than 32 million prescriptions for levothyroxine were issued in England in 2018, NHS figures show.
The advice in the BMJ aims to give guidance based on new and best evidence.
The researchers looked at data from 21 trials, involving more than 2,000 patients (many over the age of 65) to reach their conclusions.
They say almost all adults with mild or "subclinical" hypothyroidism will not benefit from hormone treatment.
Taking a pill and attending lifelong check-ups is burdensome and there is "uncertainty" over potential harms, they add.
Current UK guidelines acknowledge that many patients will not need treatment but add that for some trying daily pills may be worthwhile.
Dr Mark Vanderpump, from the Society for Endocrinology, said: "It can be reasonable to try the tablets for a few months and see how the patient feels.
"You do not have to commit someone to lifelong treatment."
Prof Simon Pearce, from Newcastle University, said: "Thyroid disease is being overtreated currently but it's premature to make a recommendation not to treat young people on the basis of the available evidence. Some will feel better on treatment."
Prof Helen Stokes-Lampard, who chairs the Royal College of GPs, said: "Thyroid hormones are powerful drugs and GPs will only ever prescribe them if we think they are of genuine benefit to the person sitting in front of us, particularly as it usually means taking the tablets and being monitored in the long term.
"If evidence shows that they are not going to be of benefit to our patients, it is important that we know this and that it is reflected in the clinical guidelines that inform our decision-making.
"The authors make a powerful case based on emerging evidence and it is important that this new research is taken on board as clinical guidelines are updated and developed, in the best interests of our patients."