Weight gain, fatigue, brain fog. These are all hallmark symptoms of hypothyroidism, a condition in which the thyroid, a small gland at the front of your neck, slows down or stops making the hormone thyroxine, which helps keep your metabolism up to speed.

If you are diagnosed with hypothyroidism, a doctor may give you a prescription for levothyroxine (Levoxyl, Synthroid and generics). This medication contains synthetic thyroid hormone, which can correct the hormone deficiency and improve your symptoms.

“Unfortunately, these unwelcome symptoms are not specific for hypothyroidism, but are also part and parcel of the natural aging process and can affect people with normally functioning thyroid glands,” says Consumer Reports’ chief medical adviser Marvin M. Lipman, an endocrinologist.

Nevertheless, people who are desperate to lose weight and feel more energetic turn to supplements that are marketed to boost metabolism and energy “naturally.” But experts warn that taking these thyroid supplements is a bad idea. Here’s why:

Thyroid supplements might contain unknown quantities of actual thyroid hormones.

It isn’t possible to know from reading the label whether a supplement contains thyroid hormones, but a 2013 study found that 9 out of 10 supplements marketed for thyroid health and support contained real hormones. Four of those that tested positive listed the ingredient “bovine thyroid tissue,” which may naturally contain hormones.

But five supplements that tested positive listed only herbal ingredients, such as ashwagandha, guggul and Coleus forskohlii. “Since plants cannot produce the hormones the researchers found, thyroid hormones from an animal or synthetic source must have been deliberately added to these supplements,” says Consumer Reports’ senior scientist Michael Hansen.

That is concerning because healthy thyroid hormone levels are very precise, and taking supplements that contain these hormones can alter those levels in unpredictable ways. “Thyroid hormone levels even slightly above or below where they should be can lead to health complications,” Lipman says. “For example, taking more thyroxine than you need can cause erratic heartbeats and bone thinning.”

They can contain iodine.

We need only 150 micrograms, or mcg, of iodine per day in our diet, according to the Institute of Medicine. “That tiny amount of iodine enables the thyroid to manufacture just the right amount of the thyroid hormone thyroxine,” Lipman says.

But ingesting excess iodine can cause health problems. “Even a slight excess of iodine can cause your thyroid to go into overdrive and produce excess thyroxine,” Lipman says. “An overactive thyroid can cause sudden weight loss, a rapid or irregular heartbeat, sweating, and nervousness or irritability.”

Conversely, too much iodine can cause the thyroid to slow down or even stop producing hormones in certain people — the opposite effect of what many people hope these supplements will do for them. That can result in weight gain and fatigue, and can increase your risk of cardiovascular disease, Lipman says.

Thyroid supplements may have kelp in them.

Kelp, a type of seaweed that is often marketed for thyroid health, is loaded with iodine. For example, a serving (one drop) of Liquid Kelp, a dietary supplement promoted for “thyroid gland support,” contains 800 mcg of iodine.

“Most people get enough iodine from their regular diet,” Lipman says. But if you take a supplement that contains kelp, plus a multivitamin such as GNC Women’s Ultra Mega One Daily containing 150 mcg of iodine and also use iodized salt that contains 400 mcg of iodine per teaspoon, it’s easy to consume far more iodine than your thyroid needs — and far more than is healthy.

They might also contain cow “glandulars.”

Glandular organs — such as thyroid, liver, pancreas, heart and spleen — can be found on the ingredients list of some thyroid and metabolic support supplements. For example, Natural Sources’ Raw Thyroid supplements contain raw thyroid, adrenal, pituitary and spleen bovine tissue. But Consumer Reports’ experts say that ingesting such ingredients is not wise. “Supplements that contain pituitary or brain products from cows could theoretically pose a risk for Creutzfeldt-Jakob, a rare disease that occurs in humans and causes brain tissue to degenerate rapidly,” Hansen says.

Thyroid supplements can hinder treatment for a thyroid condition.

Thyroid problems can be diagnosed easily through blood tests, but taking supplements that can alter the level of thyroid hormones in your blood can mask thyroid issues. “If your doctor can’t establish how much thyroid hormone your body needs, he can’t prescribe the correct amount, and that can cause health problems,” Lipman says.

In addition, Duffy MacKay, senior vice president for science and regulatory affairs at the Council for Responsible Nutrition, a trade association that represents the dietary supplement industry, acknowledges that thyroid supplements can interact with prescription medications.

Bottom line: Do not take thyroid supplements. If you suspect that you have a thyroid condition, head to your doctor’s office instead of the vitamin or natural-foods store.

Copyright 2016. Consumers Union of United States Inc.

For further guidance, go to www.ConsumerReports.org/Health, where more detailed information, including CR’s ratings of prescription drugs, treatments, hospitals and healthy-living products, is available to subscribers.