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 that helps keep your metabolism up to speed. If you are diagnosed with hypothyroidism (by a blood test), a doctor may give you a prescription for levothyroxine (Levoxyl, Synthroid, and generics). This medication contains synthetic thyroid hormone that 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 endocrinologist and Consumer Reports' chief medical adviser Marvin M. Lipman, M.D. “Less than 2 percent of people have hypothyroidism,” Lipman says. 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:

1. Thyroid Supplements Might Have Actual Thyroid Hormones

It isn’t possible to know if a supplement contains thyroid hormones from reading the label, but a 2013 study published in the journal Thyroid found that nine 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 might 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, Ph.D.

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 heart beats and bone thinning.”

It’s also impossible to know how much thyroid hormone a supplement might contain. Several of the supplement samples the researchers tested contained doses of thyroid hormone that were higher than 25 mcg—the lowest dose of levothyroxine available by prescription. And one contained more than 90 mcg—slightly less than a doctor would prescribe for a patient whose thyroid gland had been removed.

2. They Can Contain Iodine

We need only 150 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. It is particularly worrisome for people with thyroid nodules—bumps on the thyroid glands that can be very small and often go unnoticed. “If you have a nodular thyroid—and chances are that 50 percent of us will develop one or more nodules by the time we’re 60—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, studies also show that too much iodine (more than around 400 mcg per day) 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.

Yet many supplements contain more than 150 mcg of iodine. For example, both Enzymatic Therapy’s Metabolic Advantage and Whole Foods’ Thyroid Complete contain 200 mcg per two-capsule serving. Follow the recommendation on the labels and swallow two capsules three times a day and you’ve ingested 600 mcg of iodine—enough to cause health problems in certain people if taken long term. 

3. Thyroid Supplements Might 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—or that is healthy. 

4. 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 our experts say that ingesting ingredients like these 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. The disease has been linked to eating material from cows infected with mad cow disease,” Hansen says.

Prions, the agents that spread mad cow and Creutzfeldt–Jakob disease, are most commonly found in the brains of cows, but there is evidence that if an animal has an infection, you can also find prions in their spleens, kidneys, and other glandular organs,” Hansen says. “Those prions are far harder to kill than bacteria or viruses, so processing or treating the organs before adding them to supplements won’t necessarily eliminate the risk,” he says.

5. 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. And because supplements aren’t regulated the same way drugs are, they can contain varying amounts of active ingredients. “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.

The Council for Responsible Nutrition, a trade association that represents the dietary supplement industry, acknowledges that thyroid supplements can interact with prescription medications. And, says Duffy MacKay, N.D., CRN’s senior vice president for science and regulatory affairs, “It is important to talk to your doctor before starting thyroid supplements.”

Yet Stephanie Lee, M.D., Ph.D., director of the Thyroid Health Center at Boston Medical Center and a spokeswoman for the Endocrine Society says a well-informed physician would never advise a patient to purchase an over-the-counter thyroid support supplement. “If the thyroid functions were abnormal, we would prescribe the FDA-approved medication because, among other issues, there’s no evidence that thyroid support products improve thyroid function, and they can cause health complications and confuse a diagnosis,” Lee says.

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. Getting the right diagnosis and treatment is the key to combating weight gain, exhaustion, and brain fog.