A major recall of a popular thyroid medication has been issued after inspectors found the pills may not be fully effective.

Major Pharmaceuticals, one of the largest suppliers of generic medicines in the US, issued the alert for its levothyroxine sodium tablets after they were found to contain a 'subpotent' dose of the drug.

It means that the pills may not contain as much of the drug as listed on the label, suggesting users could receive a lower dose than prescribed.

The drugs were sold nationwide at major retailers in 10-count blister packs that were available either individually or in cartons containing 100 tablets.

They were available in doses 25, 50, 75, 88, 112 and 150 micrograms and have expiration dates from July 2026 to January 2027.

The FDA said it was a Class II recall, suggesting the drug may cause temporary or medically reversible effects.

No injuries related to the recall have been reported to date. 

In previous cases, patients who have medications that have been recalled have been advised to contact their doctor. Patients are advised not to stop taking a medication they have been prescribed without first speaking to their physician.

More than 22 million prescriptions for levothyroxine are written in the US every year, which is used to treat thyroid issues.

Patients who receive the drug include those with hypothyroidism, a condition where the gland does not produce enough of the thyroid hormone, or those suffering from other complications from the gland.

The FDA revealed the recall online.

A full list of the drugs recalled and their lot codes is available on the agency's website. 

The drugs were distributed by Cardinal Health and Major Pharmaceuticals across the country. 

It was not clear how the error occurred, but this can happen if there is a problem with a machine that is manufacturing the drugs.

The thyroid is a small, butterfly-shaped gland at the front of the neck that releases hormones that help to control the speed of the metabolism, or how fast the body uses energy.

An estimated 20 million Americans suffer from complications with the organ, however. 

The most common form is hypothyroidism, or an underactive thyroid, which is five to eight times more likely to occur in women than men.

Levothyroxine is a man-made form of the hormone and works by raising levels in the body, helping to ease symptoms. 

Warning signs of this include a faster than normal heart rate, feeling fatigued, unexplained weight gain and a depressed mood.

Those who are female, have a family history of the disease or are over 60 years old are at higher risk of the condition, experts say.

It is not clear why women are at higher risk, but experts say it may be linked to a difference in how the immune system functions. 

It can be diagnosed using blood tests to check levels of hormones released by the thyroid.

Doctors may prescribe medications or suggest surgery to treat the condition.