A blood test is used to detect excess prolactin. If prolactin levels are high, more tests are usually done to check blood levels of thyroid hormone. Normal thyroid hormone levels rule out hypothyroidism as a cause of hyperprolactinemia. Doctors will also ask about other conditions and medication use, and rule out pregnancy.
If a prolactinoma is suspected, an MRI (magnetic resonance imaging) of the brain and pituitary is often the next step. Using a special machine that creates images of body tissues, an MRI can reveal a pituitary tumor and show its size.