An endocrinologist is a physician with extensive training in understanding, diagnosing and treating conditions related to the endocrine system. The endocrine system regulates the balance of hormones. Conditions of the endocrine system involve an over-abundance, or deficiency of a certain hormone. While there is a range when it comes to the amount of a hormone that is deemed normal in a human, these specialists determine whether a person's amount of hormone is indicative of a health concern.

A gastroenterologist is a specialist in diagnosis and treatment of conditions involving the digestive/gastrointestinal (GI) tract. These doctors are experts on how food moves through the digestive system and is chemically broken down, with nutrients being absorbed and waste excreted. You might see this kind of doctor if you are experiencing any number of stomach issues, some of which might be severe diarrhea, irritable bowel syndrome, hemorrhoids, ulcers, acid reflux, Crohn's disease and more.

An internist is a physician who focuses on the diagnosis and treatment of conditions that affect the adult population—both acute and chronic.These doctors are often who adults see as their primary physicians because they treat a broad range of illnesses that do not require surgical or specialist interventions. They also work to help a patient maintain optimal health in order to prevent the onset of disease.In addition to treating the common cold and flu, internists also treat chronic diseases like diabetes and heart disease.

An ophthalmologist has the training to do much more than just prescribe glasses. They are physicians specially trained to diagnose and treat disorders of the eyes and vision. These doctors are experts on the complicated anatomy of the eye and are trained to treat eye diseases through both medical and surgical methods.Some common conditions that ophthalmologists treat are cataracts, glaucoma, strabismus, diabetic retinopathy, macular degeneration and amblyopia. In addition, ophthalmologists can provide prescriptions for eye glasses and contact lenses and perform LASIK surgery and other corrective surgeries for refractive errors like myopia (near-sightedness), hyperopia (far-sightedness), astigmatism and presbyopia.