Hashimoto's thyroiditis is an autoimmune disorder where the immune system mistakenly attacks the thyroid gland, leading to inflammation. This condition often results in an underactive thyroid, known as hypothyroidism, which can cause symptoms like fatigue, weight gain, and sensitivity to cold. The exact cause of Hashimoto's is not fully understood, but it may involve genetic and environmental factors. Diagnosis typically involves blood tests to check levels of thyroid hormones and antibodies. Treatment usually includes hormone replacement therapy to manage symptoms and maintain normal thyroid function.