This chapter describes the thyroid gland, its associated hormones, and metabolism. Thyroid function is regulated most importantly by thyroid-stimulating hormone (TSH), also called thyrotropin. In turn, TSH secretion from the anterior pituitary is stimulated by the hypothalamic factor thyrotropin-releasing hormone (commonly abbreviated as TRH). It investigates genetic thyroid disorders, thyroid hormone resistance, autoimmune hypothyroidism, and Graves’ disease. The chapter lists the modalities of thyroid imaging, including ultrasound and radionuclide imaging (RNI), and describes indications of endocrine disease, like goitre, nodules, and malignancy. It describes thyroid hormone homeostasis and conditions affecting homeostasis. It also describes clinical features, causes, and management for disorders including hypothyroidism, hyperthyroidism, thyroiditis, thyroid nodules, and cancer.