Acromegaly with normal basal growth hormone levels.

Background: The most common cause of acromegaly is excess of growth hormone (GH) secretion.

Methods: We report a 42-year-old male patient, who had become acromegalic over the past 5 years. There were no visual changes or change in sexual function, no gynaecomastia or galactorrhoea. Both CT and MRI scans showed a large mass measuring 2.5 x 2.5 x 3.5 cm, originating from the sella turcica and extending into and totally filling up the sphenoid sinus with diffusely invasive features.

Results: Basal serum GH level was within normal range, but insulin-like growth factor 1 (IGF-1) was elevated with slightly increased prolactin (PRL) and impaired GH secretory regulation as well. A pituitary adenoma was partially removed through transsphenoidal microsurgery. Pathology confirmed a mammo-somatotrophic adenoma but immunocytochemistry study of the tumour showed only positivity for PRL but not GH.

Conclusions: When acromegaly occurs without GH level elevation, one should pay attention that: 1) IGF-1 might be the cause of the clinical feature of acromegaly; 2) The tumour might undergo morphological transformation; and 3) Hyperinsulinemia or GH receptor antibody formation could also be the cause of the acromegalic appearance.