Your thyroid is a butterfly-shaped gland that sits in your neck. Normally, it produces hormones that regulate a variety of bodily processes, including your heart rate and energy metabolism. It receives messages from your brain's pituitary gland in the form of thyroid-stimulating hormone (TSH). When the thyroid gets too much or not enough TSH, its own hormone production can get out of whack.It's the most common form of hypothyroidism , meaning the thyroid is underactive and isn't responding to the pituitary's TSH. That's because, with Hashimoto's, your body has created antibodies that are actually attacking the thyroid, preventing it from doing its very important job. The symptoms of Hashimoto's usually come on gradually over years and can include weight gain, fatigue, abnormally heavy periods, and feelings of depression. It may be up to 10 times more common in women than men.In some cases, your hormone levels will be normal in early phases, and your doctor might suggest an observational approach. But many patients end up taking synthetic hormones , which need to be taken for the rest of their lives.Both diagnosis and treatment are often complicated processes. For example, there's some disagreement among experts about what the ideal level of TSH is, and your levels can vary from test to test. So if you're concerned about symptoms that suggest a thyroid problem (issues with energy levels or unexplained weight changes are big ones), it's extremely important to work with your doctor and, if needed, an endocrine specialist to figure out the best course of action for you.