Hypothyroidism is a disorder where the thyroid gland does not produce enough thyroid hormone. It can be caused by autoimmune disease, over treatment of hyperthyroidism, thyroid surgery, radiation therapy, or certain medications. Common symptoms include fatigue, weight gain, dry skin, and joint and muscle pain. Diagnosis is based on blood tests measuring thyroid stimulating hormone and thyroxine levels. Treatment involves lifelong thyroid hormone replacement therapy to replicate normal thyroid function. Diet and lifestyle changes like avoiding iodine and processed foods can also help manage the condition.