Disentangling secretory ambiguity and the limitations of classical thresholds in defining prolactinomas

Differentiating prolactinomas from non-functional pituitary adenomas (NFPAs) with stalk effect relies on biochemical thresholds that assume a linear relationship between tumor size and prolactin secretion. We applied a pathology-informed Prolactin Secretory Efficiency (PSE) framework to characterize diagnostic overlap between prolactinomas and NFPAs and evaluate the limitations of current prolactin thresholds.

Retrospective study of 425 patients undergoing first-time surgery for pituitary adenomas: 115 pathology-confirmed lactotroph adenomas, 291 NFPAs, and 19 mammosomatotrophs. Patients with clinical or biochemical evidence of ACTH- or GH-secreting tumors were excluded. PSE was calculated as the serum prolactin-to-tumor volume ratio [Formula: see text]). Primary outcomes included PSE-based group separation and diagnostic accuracy of the 200 ng/mL threshold.

While no NFPA exceeded 200 ng/mL, 42% of pathology-confirmed macroprolactinomas and 49% of macro-NFPAs fell within the diagnostic grey zone (ULN < prolactin < 200 ng/mL). PSE achieved superior group separation versus raw prolactin, yet a paradox emerged: high-efficiency stalk effect in some NFPAs produced PSE values exceeding those of low-efficiency macroprolactinomas.

Rigid biochemical thresholds fail to capture the full spectrum of lactotroph adenoma biology. Significant secretory ambiguity exists for macroadenomas below 200 ng/mL, creating risk of misclassification. D2-agonist trials may serve as a low-risk diagnostic probe in this grey zone, ensuring low-efficiency prolactinomas receive appropriate first-line medical management.