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Differentiating prolactinomas from non-functioning sellar masses causing hyperprolactinemia due to stalk effect is a common diagnostic challenge. While both can result in elevated serum prolactin, accurate discrimination is essential for appropriate management.

To determine the upper limit of serum prolactin in non-functioning sellar masses attributable to stalk effect.

This retrospective study was conducted at a tertiary care center in South India from January 2015 to December 2024. Patients with both non-pituitary sellar masses and pituitary tumors with negative prolactin immunohistochemistry were included. Patients with functioning pituitary adenomas, hyperprolactinemia-inducing drugs, chronic kidney disease, severe hepatic dysfunction, and PCOS were excluded. The primary objective was to determine the upper limit of serum prolactin levels attributable to stalk effect in patients with non-functioning sellar masses. Preoperative serum prolactin levels were measured using a chemiluminescent immunoassay. The 97th percentile value was taken as the upper limit.

Of 288 cases of non-functioning sellar masses, 57 met inclusion criteria. Most of them (87.7%, 50/57) were >1 cm, with 68% (39/57) classified as pituitary adenomas-49% (19/39) being gonadotroph adenomas and 41% (16/39) null cell adenomas. Hyperprolactinemia was observed in 32% of patients. The median serum prolactin level was 14.8 ng/mL, and the 97th percentile was 70 ng/ml. No significant correlation was found between tumor size and serum prolactin level (r = 0.13, P = 0.35). Gender did not significantly affect serum prolactin levels. A serum prolactin threshold of 70 ng/mL was identified, above which non-functioning sellar masses are highly unlikely.

In patients with non-functioning sellar masses, serum prolactin levels rarely exceed 70 ng/ml. This threshold may serve as a useful diagnostic marker to distinguish stalk effect from prolactinomas. Prospective validation in larger cohorts is warranted.