Galactorrhea: This condition is characterized by the spontaneous flow of milk from the breast, unassociated with childbirth or nursing. It can be caused by various factors including hormonal imbalances, certain medications, or issues with the pituitary gland. The discharge is often bilateral and can be yellowish in color.

Ductal Ectasia: This condition involves the widening of the milk ducts in the breast, which can cause a thick, yellowish or greenish discharge. It's more common in older women and can be associated with periductal mastitis.
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 sometimes the discharge can appear yellowish due to the presence of pus or other cellular debris.
Breast Abscess: Although typically associated with pain and swelling, a breast abscess can occasionally present with a purulent discharge that may appear yellow.

Breast Cancer: While less common, certain types of breast cancer, such as ductal carcinoma in situ (DCIS) or invasive ductal carcinoma, can present with nipple discharge. Although the discharge is more often bloody or serous, it's crucial not to miss this diagnosis due to its potential severity.
Pituitary Tumor: A prolactinoma, a type of pituitary tumor, can cause galactorrhea. It's essential to diagnose and treat pituitary tumors due to their potential to cause significant morbidity.

Thyroid Disorders: Certain thyroid conditions can lead to hyperprolactinemia, resulting in galactorrhea. While not the first consideration, thyroid function should be evaluated in the workup of galactorrhea.
Hypothalamic Disorders: Rare conditions affecting the hypothalamus can disrupt normal hormonal balances, potentially leading to galactorrhea. These are less common but should be considered in the appropriate clinical context.