Thyroid disease refers to a range of disorders affecting the thyroid gland, a small butterfly-shaped gland in the neck that regulates metabolism, energy levels, and hormone balance. Common conditions include hypothyroidism (underactive thyroid), hyperthyroidism (overactive thyroid), goiter, and thyroid nodules. Symptoms vary but may include weight changes, fatigue, mood swings, and temperature sensitivity. Treatment depends on the type of thyroid disorder and may involve medication, lifestyle changes, or surgery.