The term multiple endocrine neoplasia (MEN) encompasses several distinct syndromes featuring tumors of endocrine glands, each with its own characteristic pattern. In some cases, the tumors are malignant, in others, benign. Benign or malignant tumors of nonendocrine tissues occur as components of some of these tumor syndromes.

MEN syndromes are inherited as autosomal dominant disorders.

Although not officially categorized as multiple endocrine neoplasia syndromes, Von Hippel-Lindau disease and Carney complex are two other autosomal dominant endocrine tumor syndromes with features that overlap the clinical features of the MEN syndromes. Although not transmitted in the germline, McCune-Albright syndrome is a genetic disorder characterized by endocrine neoplastic features involving endocrine glands that overlap with those involved in MEN1 or MEN2.

The most commonly used diagnostic criteria and definition of CFS for research and clinical purposes were published by the United States Centers for Disease Control and Prevention (CDC). The CDC currently recommends the following criteria for diagnosis:

1. Significantly lowered ability to participate in activities that were routine before the onset of the condition, and persisting more than six months

2. Physical or mental activity causes worsening symptoms that would not have been problematic before the onset of the condition, (post-exertional malaise (PEM))

3. Sleep problems

Additionally, one of the following symptoms must be present:

- Difficulty with thinking and memory

- Worsening of problems with standing or sitting

Other common symptoms may include:

- Muscle pain, joint pain, and headache pain

- Tender lymph nodes in the neck or armpits

- Sore throat

- Irritable bowel syndrome

- Night sweats

- Sensitivities to foods, odors, chemicals, or noise

The CDC proposes that persons with symptoms resembling those of CFS consult a physician to rule out several treatable illnesses: Lyme disease, "sleep disorders, major depressive disorder, alcohol/substance abuse, diabetes, hypothyroidism, mononucleosis (mono), lupus, multiple sclerosis (MS), chronic hepatitis and various malignancies." Medications can also cause side effects that mimic symptoms of CFS. Central sensitization, or increased sensitivity to sensory stimuli such as pain have been observed in CFS. Sensitivity to pain increases post-exertionally, which is opposite to the normal pattern.

Studies have mixed results as to whether a gradual onset or sudden onset is more frequent.