Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1164 page)

   Elevated levels in patients with chronic renal failure, cirrhosis, and hyperthyroidism.

   Proinsulin >30% of serum insulin after overnight fast suggests insulinoma.

   Proinsulin is increased in factitious hypoglycemia due to sulfonylurea.

   Proinsulin is increased in familial hyperproinsulinemia—heterozygous mutation affecting cleavage of proinsulin leading to secretion of excess amounts of proinsulin.

   Type 2 DM.

   Limitations

   Proinsulin may also be increased in renal disease.

   Elevation of proinsulin: insulin ratio correlates with a decreased acute illness response to glucose in patients with type 2 DM.

PROLACTIN

   Definition

   Prolactin is a single-chain polypeptide composed of 198 amino acids and is secreted by the anterior cells of the pituitary gland. Prolactin secretion is controlled by the hypothalamus primarily through the release of prolactin-inhibiting factor (dopamine) and prolactin-releasing factor (serotonin). TRH stimulates prolactin secretion and is useful as a provocative test to evaluate prolactin reserves and abnormal secretion of prolactin by the pituitary. The primary physiologic function of prolactin is to stimulate and maintain lactation in women.

   
Normal range:

   Males: 2.64–13.13 μg/L

   Females <50 years (premenopausal): 3.34–26.72 μg/L