Galactorrhea due to ductal ectasia: This condition is characterized by a milky discharge from the breast, often unilateral, and can occur in the absence of prolactin imbalance. Ductal ectasia involves the widening of the milk ducts, which can lead to the secretion of a milky fluid. Given the normal prolactin levels and the absence of any mass on ultrasound, this diagnosis is plausible.

Intraductal papilloma: These are small, benign tumors that grow in the milk ducts of the breast. They can cause a bloody or serous discharge but can occasionally present with a milky discharge. The normal ultrasound might not detect smaller papillomas, making this a possible diagnosis.
Breast ductal hyperplasia: This condition involves an overgrowth of cells within the breast ducts and can lead to various types of nipple discharge, including milky white.

Prolactinoma (with normal prolactin levels): Although prolactin levels are reported as normal, it's crucial to consider that prolactinomas (a type of pituitary tumor) can sometimes present with normal or only slightly elevated prolactin levels, especially if the tumor is small or if the blood sample was taken at a time when prolactin levels were not peaked.
Breast cancer: While less common, certain types of breast cancer, like intraductal carcinoma, can present with nipple discharge. The absence of a mass on ultrasound does not entirely rule out cancer, as some tumors may not be visible or may be located in areas difficult to assess with ultrasound.

Thyroid disorders: Hypothyroidism can sometimes cause an increase in TRH (thyrotropin-releasing hormone), which can stimulate prolactin release, potentially leading to galactorrhea. However, this would typically be associated with other symptoms of hypothyroidism.
Neurogenic causes: Certain conditions affecting the spinal cord or brain can lead to galactorrhea due to the disruption of normal neuroendocrine pathways. These are rare and usually associated with other neurological symptoms.