Your thyroid produces hormones that regulate your metabolism, skin, and heart. When the thyroid doesn’t make enough hormones, your metabolism can suffer. You’ll likely notice weight gain even though you haven’t changed your diet or workout routine. Other symptoms of a sluggish thyroid include constipation, hair loss, dry skin, and feeling cold all the time. You may also feel tired frequently despite proper rest. Stimulants like caffeine can make the problem worse.