Hyperprolactinemia is a known side effect of antipsychotic agents, and this patient's hyperprolactinemia is likely due to risperidone. Antipsychotics block dopamine and decrease inhibition of prolactin release at the pituitary, causing hyperprolactinemia. Stopping the medication can reverse the hyperprolactinemia but should only be done under the supervision of a psychiatrist. The patient's risperidone should not be discontinued unless her psychiatrist is consulted.

Hypothyroidism can cause hyperprolactinemia when uncontrolled, but her hypothyroidism is well treated. She has no symptoms of hypothyroidism, and her thyroid function tests are normal, so uncontrolled hypothyroidism is not the cause of her hyperprolactinemia.

Antipsychotic agents cause hyperprolactinemia by blocking dopamine, but lithium does not cause hyperprolactinemia.

Although the patient has a likely explanation for her hyperprolactinemia, a prolactinoma is still possible. Risperidone can cause prolactin levels above 200 ng/mL (200 µg/L), so her level of 102 ng/mL (102 µg/L) is not unreasonable. Remeasuring the prolactin level following discontinuation of the drug is recommended before further evaluation for a pituitary adenoma; the patient's psychiatrist should be consulted before withholding the risperidone for testing. A patient with hyperprolactinemia without a clear secondary or drug-induced cause should be assessed by an imaging study (preferably, MRI of the pituitary gland) to a exclude pituitary lesion.