Masked hyperprolactinemia: tumor-derived factors inhibiting prolactin secretion caused by pituitary-stalk damage.

Tumor-induced secondary hyperprolactinemia in patients with non-prolactin (PRL)-secreting pituitary tumors has traditionally been ascribed to pituitary stalk damage. We conducted a retrospective analysis of secondary hyperprolactinemia in 106 patients who underwent surgery for non-PRL-secreting pituitary adenoma. The incidence of hyperprolactinemia was evaluated, and pituitary-stalk damage was assessed radiographically using MRI (size of tumor and extension type) and endocrinologically by monitoring hormonal function using a provocation test. The effect of a tumor-derived intrasellar factor, leukemia inhibitory factor (LIF), on hyperprolactinemia was also investigated. Hyperprolactinemia was observed in 31 of the 106 (29.2%) patients. It was not correlated with either physical stalk compression or endocrinological dysfunction. However, LIF expression was negatively correlated with the incidence of secondary hyperprolactinemia (p<0.01). Although secondary hyperprolactinemia might be caused by pituitary stalk damage, it is possible that LIF masks the effect.