Gigi Hadid got honest about a bunch of facets of her life earlier this week at a Reebok panel — including her health. In particular, she shed some light on what it was like to prepare for the Victoria's Secret Fashion Show while also dealing with a thyroid disease. "My metabolism actually changed like crazy this year," she explained . "I have Hashimoto's disease. It's a thyroid disease, and it's now been two years since taking the medication for it."What is Hashimoto's? The condition occurs when your body's immune system basically begins to destroy your thyroid gland. The thyroid, which is often described as a "butterfly-shaped" gland in your neck, regulates the release of hormones that play a role in a ton of bodily processes including breathing, metabolism, the menstrual cycle, and temperature regulation. So if something goes wrong there, the symptoms can be far-reaching.You might have heard of hyperthyroidism, a condition where the thyroid is overactive — it's often associated with rapid heartbeat, sweating, and weight loss. But in the case of Hashimoto's, as well as other forms of hypothyroidism , the thyroid becomes underactive, and the symptoms that result include fatigue, heavy periods, depression, and sometimes weight gain or difficulty losing weight.As with many hormonal issues, diagnosis and treatment of Hashimoto's can be a confusing process. Because many of the symptoms tend to come on gradually and can be caused by other illnesses, it often takes a while for patients (and their doctors) to figure out what's really going on. Plus, experts don't always agree on the exact level of hormones that indicate a problem and, therefore, a diagnosis.When it comes to treatment , your plan usually depends on the severity of your symptoms and your hormone levels. Initially, your symptoms may be mild and your hormone levels may not be worrying, so your doctor may suggest simply keeping an eye on things. But once things get really out of whack, most patients end up taking synthetic hormones for the rest of their lives. And in really severe cases, surgery may be the best option.Though thyroid conditions are relatively rare, they're more common than you'd think — and they're up to 10 times more common for women than men. So if you think you may have a thyroid issue, check in with your doctor to figure out what's up.