Which condition is characterized by a lack of insulin or insufficient use of insulin?

The condition characterized by a lack of insulin or insufficient use of insulin is diabetes mellitus. This metabolic disorder primarily involves the body's inability to produce enough insulin or use it effectively, leading to elevated blood glucose levels.

In type 1 diabetes, the body does not produce insulin at all due to autoimmune destruction of pancreatic beta cells, while in type 2 diabetes, the body either does not produce enough insulin or the cells become resistant to insulin's effects. Both types result in poor glucose regulation, leading to various complications if left untreated.

Other conditions listed do not directly relate to insulin deficiencies. For example, thyroiditis involves inflammation of the thyroid gland and does not affect insulin levels directly. Polycystic ovary syndrome (PCOS) may involve insulin resistance but is primarily related to hormonal imbalances that affect ovarian function. Hypothyroidism is related to an underactive thyroid but does not relate directly to insulin production or use. Thus, diabetes mellitus is the clear option when looking for a condition marked by issues related to insulin.