In postmenopausal women with normal pituitary function, LH and FSH levels should be appropriately elevated. Loss of this elevation may be one of the earliest signs of pituitary dysfunction in this group.
TSH, free T4
Central hypothyroidism (low T4, low/normal TSH). Rarely, a modestly elevated TSH (7 to 15) with a low T4 may occur in central hypothyroidism.
TSH-producing tumour (high T4, normal/elevated TSH). The differential diagnosis includes central thyroid hormone resistance or issues with the TSH or T4 assay/measurements.
Patients with primary hypothyroidism (low T4, very high TSH) may present with a physiologic pituitary hyperplasia and a moderately elevated prolactin level, both of which should resolve after adequate thyroid hormone replacement.
ACTH, Cortisol (8 am)
Secondary AI (low cortisol, low/normal ACTH)
Cushing's disease cannot be ruled out by a cortisol measurements alone. If clinically suspected, it should be ruled out with a dexamethasone suppression test, 24-hour urinary free cortisol measurement, and/or midnight salivary cortisol levels.
Prolactinoma (high prolactin)
Very high levels of prolactin may interfere with the assay and result in a spuriously lower result ("the hook effect"). This can be factored out by measuring prolactin with serial dilution
Prolactin-producing adenomas > 1 cm in size are associated with prolactin levels > 100-200 mcg/L (up to 1000's). Large adenomas associated with a more moderate prolactin elevation (e.g., ~ 50-100 mcg/L) are more likely to be nonfunctionining tumours with pituitary stalk-compression as the cause of prolactin elevation.
Very high prolactin levels may appear paradoxically low in lab measurements due to an assay interference. This can be confirmed by serial dilution.
Hyponatremia in AI or central hypothyroidism, hypernatremia in uncompensated DI
Low specific gravity in DI
Diabetes in patients with Cushing's or acromegaly
Anemia in men with hypogonadism
Hypopituitarism due to compression from a large pituitary adenoma often progresses through loss of GH, LH, FSH, TSH, ACTH, then prolactin, in that order (mnemonic: Go Look For The Adenoma Please). However, this does not necessarily apply to hypopituitarism due to an infiltrative condition (e.g., hypophysitis or sarcoidosis).