An endocrinologist is a medical doctor specializing in the endocrine system—the glands (like thyroid, pituitary, adrenal, pancreas) that produce hormones that regulate body functions like growth, metabolism, and mood. They diagnose and treat hormone imbalances and related disorders, most commonly diabetes, thyroid diseases, pituitary/adrenal gland issues, infertility, and metabolic conditions, helping restore hormonal balance through medications or other therapies.