Objective: To review the pertinent basic and clinical research describing the complex effects of excess thyroid hormone on carbohydrate metabolism. Methods: We performed a MEDLINE search of the English-language literature using a combination of words (ie, "thyrotoxicosis and diabetes," "diabetic ketoacidosis and thyroid storm," "carbohydrate metabolism and hyperthyroid," "glucose homeostasis and thyrotoxicosis") to identify key articles addressing various aspects of the thyroid's influence on carbohydrate metabolism. Results: Thyroid hormone affects glucose homeostasis via its actions on a variety of organs including increased hepatic glucose output, increased futile cycling of glucose degradation products between the skeletal muscle and the liver, decreased glycogen stores in the liver and skeletal muscle, altered oxidative and nonoxidative glucose metabolism, decreased active insulin output from the pancreas, and increased renal insulin clearance. Thyroid hormone also affects adipokines and adipose tissue, further predisposing the patient to ketosis. Conclusions: Thyrotoxicosis can alter carbohydrate metabolism in a type 2 diabetic patient to such an extent that diabetic ketoacidosis develops if untreated. Based on the current understanding of this relationship, all diabetic patients should be screened for thyroid dysfunction because correcting hyperthyroidism can profoundly affect glucose homeostasis. Similarly, patients presenting in diabetic ketoacidosis should undergo a thyroid function assessment.