Physiologic Gynecomastia: This condition is common during puberty and can persist into adulthood. The patient's history of gynecomastia since age 14, with the left side resolving and the right side persisting, along with recent weight gain, supports this diagnosis. The low estradiol level (<5.0 pg/mL) is within the normal range for males, which further supports a physiologic cause rather than a pathologic one related to hormonal imbalance.

Pseudogynecomastia: Given the patient's recent weight gain, it's possible that the perceived gynecomastia is actually pseudogynecomastia, which is an increase in fat tissue in the breast area without an actual increase in glandular breast tissue.
Idiopathic Gynecomastia: This diagnosis is considered when no underlying cause for gynecomastia can be identified. The patient's symptoms and laboratory results do not point to a specific cause, making idiopathic gynecomastia a possibility.

Testicular Cancer: Although rare, testicular cancer can produce human chorionic gonadotropin (hCG), which can stimulate estrogen production, leading to gynecomastia. It's crucial to examine the testes and consider imaging or tumor markers if there's any suspicion.
Pituitary Tumors: Tumors of the pituitary gland, such as prolactinomas, can disrupt normal hormonal balances, potentially leading to gynecomastia. The absence of galactorrhea or other symptoms does not rule out this possibility entirely.
Hypogonadism: Conditions affecting the testes' ability to produce testosterone can lead to an imbalance in estrogen and testosterone levels, causing gynecomastia. Although the patient's estradiol level is low, hypogonadism could be considered, especially if there are other symptoms of low testosterone.

Klinefelter Syndrome: A genetic condition (47,XXY) that affects male development and can lead to gynecomastia, among other symptoms. It's rare and typically diagnosed earlier in life, but it could be considered if other signs or symptoms are present.
Thyroid Disorders: Both hyperthyroidism and hypothyroidism can lead to gynecomastia, although this is rare. The patient's symptoms and laboratory results do not specifically suggest a thyroid disorder, but it could be considered in a broader differential diagnosis.
Liver Disease: Severe liver disease can lead to an imbalance in sex hormones, resulting in gynecomastia. The patient's history and laboratory results do not indicate liver disease, but it remains a rare possibility.