You may know the classic signs of trouble with your thyroid -- a butterfly-shaped gland on the front of your neck. An overactive thyroid (hyperthyroidism) makes you sweat more, feel hot, lose weight, or makes your heart beat faster. An underactive thyroid (hypothyroidism) does the opposite. You might feel cold, gain weight, and get a slower heartbeat. But there are other less obvious signs that you need to learn about.