Hashimoto's thyroiditis is the primary cause of hypothyroidism, an autoimmune disease where the immune system attacks the thyroid, leading to inflammation and eventual underactivity. Common symptoms include fatigue, cold intolerance, sluggishness, body aches, and weight gain due to slowed metabolism. The inflammation can cause an enlarged thyroid (goiter) or, over time, a shrunken and scarred gland. Thyroid nodules can also develop, which are typically less likely to be malignant when associated with this autoimmune process. Hashimoto's thyroiditis is significantly more common in women than in men and shows varying prevalence across different ethnicities. Treatment involves supplementing missing thyroid hormone and addressing any anatomical abnormalities, as the underlying immune system dysfunction cannot be directly corrected.