Hypothyroidism is a condition where the thyroid gland does not produce enough thyroid hormones. This can lead to symptoms such as fatigue, weight gain, and sensitivity to cold. It is often diagnosed through blood tests that measure hormone levels and can be treated with hormone replacement therapy. Hyperthyroidism, on the other hand, occurs when the thyroid gland produces too much thyroid hormone. This can result in symptoms like weight loss, increased heart rate, and anxiety. It is commonly caused by conditions such as Graves' disease and can be managed with medications, radioactive iodine, or surgery.