The thyroid gland develops from a median bud at base of tongue (foramen cecum). The thyroid gland has two types of secretory cells, follicular and parafollicular. The follicular cells secrete the thyroid hormones triiodothyronine (T3) and tetraiodothyronine (T4); the parafollicular or C-cells secrete thyrocalcitonin. Release of thyrotropin-releasing hormone (TRH) from hypothalamus stimulates anterior pituitary to release thyroid-stimulating hormone (TSH), which in turn stimulates thyroid to release T3 and T4. This chapter describes various thyroid function tests, thyroid imaging, fine-needle aspiration cytology, thyroid autoantibodies, core biopsy, and frozen section biopsy. The sub concepts of hypothyroidism, goiter, thyrotoxicosis, thyroid neoplasms, differentiated thyroid carcinoma, anaplastic carcinoma, medullary carcinoma, malignant lymphoma, solitary thyroid nodule, and thyroiditis are explained. Also, operative steps of thyroidectomy, ectopic thyroid, thyroglossal cyst, and thyroid eye disease are described.