Thyroid hormone is essential for the regulation of tissue metabolism. Thyrotropin releasing hormone is a tripeptide which is produced in the parvocellular region of the paraventricular nuclei of the hypothalamus. A wide variety of medications which may be used during critical illness can affect thyroid hormone physiology and the interpretation of thyroid function tests. Hyperthyroidism is the consequence of excessive thyroid hormone action. Hypothyroidism results from under secretion of thyroid hormone from the thyroid gland. It can be classified as primary, secondary and tertiary. The most common cause of primary hypothyroidism is chronic autoimmune thyroiditis. Other causes include surgical removal of the thyroid gland, thyroid gland ablation with radioactive iodine, infiltrative diseases, iodine deficiency, congenital enzymatic defects which affect thyroid hormone synthesis and drug-induced effects. Secondary and tertiary hypothyroidisms are usually due to pituitary and hypothalamic disorders respectively.