Hyperthyroidism is caused by conditions like Graves' disease, multinodular goiter, solitary nodules, and thyroiditis. Rare causes include TSH-induced hyperthyroidism and follicular carcinoma. Clinical features include goiter, weight loss, increased appetite, diarrhea, palpitations, angina, dyspnea, nervousness, tremors, and increased sweating. Investigations show increased T3 and T4 levels with decreased or undetectable TSH, and increased TSH receptor antibodies in Graves' disease. Management involves anti-thyroid drugs, surgery, radioactive iodine, and beta blockers.