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Stalk Effect refers to an elevated blood prolactin level (hyperprolactinemia) occurring as a result of tumors or other masses within or near the pituitary gland and stalk that block delivery of dopamine (a neurotransmitter) from the hypothalamus to the prolactin secreting cells of the pituitary. Stalk effect hyperprolactinemia is seen most often with pituitary adenomas, craniopharyngiomas and Rathke’s cleft cysts. The elevation in prolactin is typically in the range of 30 to 150 ng/ml (normal range is < 20 ng/ml). Prolactin levels greater than 200 ng/ml are almost always due to a prolactinoma.