A 23 year old man complains of persistent headache. He has noticed gradual increase in his ring size and his shoe size over the years. On physical examination, he has a peculiar deep, hollow-sounding voice and a prognathic jaw. Bedside visual field testing suggests bitemporal hemianopsia.
What initial studies are indicated?A. Serum insulin-like growth factor 1(IGF-1) and prolactin levels
The patient has excessive growth of soft tissue that has resulted in coarsening of facial features, prognathism, and frontal bossing—all characteristic of acromegaly. This growth hormone–secreting pituitary tumor will result in bitemporal hemianopsia when the tumor impinges on the optic chiasm, which lies just above the sella turcica. Growth hormone–secreting tumors are the second commonest functioning pituitary tumors (second to prolactinomas). Serum IGF-1 (insulin-like growth factor-1) level will be elevated and is usually the first diagnostic test. Since 40% of GH-producing tumors also produce prolactin, a prolactin level should be obtained as well. Growth hormone secretion is pulsatile and a single GH level is often equivocal; the GH level must be suppressed (usually with glucose) to diagnose autonomous overproduction. Dexamethasone suppression is used in the evaluation of Cushing syndrome, with partial suppressibility suggesting a pituitary cause, but this patient’s presentation strongly suggests acromegaly, not Cushing syndrome. Once GH overproduction is documented, an MRI scan of the pituitary will show the size and extent of the tumor (most are macroadenomas > 1 cm). The lateral skull film is insufficiently sensitive for this purpose. Growth hormone stimulation tests (insulin-induced hypoglycemia, arginine plus GHRH) may be used to diagnose growth hormone deficiency, but would not be useful to diagnose GH overproduction, where a suppression test should be used.