What condition is characterized by an inability to regulate glucose levels?

Diabetes mellitus is characterized by an inability to regulate glucose levels effectively. This condition arises from either insufficient insulin production by the pancreas or the body's cells not responding effectively to insulin, which is necessary for glucose to enter the cells for energy. In diabetes, there are elevated levels of glucose in the bloodstream, leading to various complications if not managed properly.

Hypoglycemia refers to low blood sugar levels, which is not the same as the inability to regulate glucose levels overall but rather a specific condition of having too little glucose. Hypertension relates to high blood pressure and does not involve glucose regulation. Hyperthyroidism is an overactive thyroid condition that influences metabolism but does not directly pertain to glucose regulation like diabetes mellitus does. Thus, diabetes mellitus stands out as the correct answer due to its direct linkage to challenges in maintaining glucose homeostasis.