The diagnosis of hypopituitarism, defined as deficient secretion of one or more pituitary hormones because of pituitary or hypothalamic disease, is made by documenting subnormal secretion of these pituitary hormones in defined circumstances. Each pituitary hormone must be tested separately, since there is a variable pattern of hormone deficiency among patients with hypopituitarism.
The impetus to measure pituitary hormones is the suspicion that the secretion of one or more may be subnormal. This suspicion can be based upon the knowledge that the patient has either a lesion known to cause hypopituitarism or a symptom known to be caused by hypopituitarism. (See "Causes of hypopituitarism" and "Clinical manifestations of hypopituitarism".) The knowledge that the patient has a lesion that can cause hypopituitarism, eg, a sellar mass, is by itself sufficient reason to test for hypopituitarism because some patients with hypopituitarism have no symptoms.
For normal health, the basal secretion of corticotropin (ACTH) must be sufficient to maintain the serum cortisol concentration within the normal range. For survival, it must increase to raise serum cortisol concentrations in times of physical stress.
Basal ACTH secretion — To test basal ACTH secretion, serum cortisol should be measured at 8 to 9 AM, and the results should be interpreted as follows:
- A serum cortisol value of ≤3 mcg/dL (83 nmol/L; normal range 5 to 25 mcg/dL [138 to 690 nmol/L]), confirmed by a second determination, is strong evidence of cortisol deficiency, which in a patient with a disorder known to cause hypopituitarism is usually the result of that disorder. Such a finding in the absence of any known cause of hypopituitarism mandates measurement of serum ACTH. A serum ACTH value not higher than normal is inappropriately low and establishes the diagnosis of secondary adrenal deficiency (ie, pituitary or hypothalamic disease). A value higher than normal documents primary adrenal insufficiency (ie, adrenal disease). (See "Diagnosis of adrenal insufficiency in adults".)
- A serum cortisol value of ≥18 mcg/dL (497 nmol/L) indicates that basal ACTH secretion is sufficient and also that it is probably sufficient for times of physical stress.
- A serum cortisol value >3 mcg/dL (83 nmol/L) but <18 mcg/dL (497 nmol/L) that is persistent on repeat determination is an indication to evaluate ACTH reserve.