Dr. Cappola says this is one of the two fatigue-related conditions she sees most often. 

Your thyroid is a small gland located at the base of your neck that regulates everything from your energy levels to your metabolism and immune function.

Thyroid issues are also common; 11 million people in the U.S. suffer from hypothyroidism.

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While a lack of energy tends to be the first and most prominent symptom of an underactive thyroid, weight gain, constipation, dry skin, and feeling cold all the time are associated symptoms to watch out for. 

“I would also look for skin with a doughy appearance,” which comes from a thyroid-related imbalance in skin chemistry, Dr. Sukol says.