Hypogonadism: This condition, characterized by low testosterone levels, can lead to all the symptoms mentioned, including fatigue, loss of muscle mass, decreased libido, and erectile dysfunction. It is a common condition that can be primary (due to testicular dysfunction) or secondary (due to hypothalamic or pituitary dysfunction).

Depression: Can cause fatigue, decreased libido, and erectile dysfunction. While it might not directly cause low muscle mass, it can lead to changes in appetite and activity levels that result in muscle loss.
Chronic Kidney Disease (CKD): Advanced CKD can lead to hormonal imbalances, including decreased testosterone, contributing to the symptoms. Additionally, CKD can cause fatigue and muscle wasting.
Diabetes Mellitus: Especially when poorly controlled, can lead to fatigue, muscle wasting, and erectile dysfunction due to neuropathy and vascular disease.
Hypothyroidism: Can cause fatigue, muscle weakness, and changes in libido. While less directly linked to erectile dysfunction, hypothyroidism can contribute to overall sexual health issues.

Pituitary Tumor: A tumor in the pituitary gland can lead to hypogonadism (due to decreased LH/FSH secretion) and can also cause fatigue and changes in muscle mass. Although less common, missing this diagnosis could have significant consequences due to the potential for the tumor to cause other hormonal imbalances and compress surrounding structures.
Sleep Apnea: Can cause significant fatigue and, through various mechanisms including hormonal changes, contribute to decreased libido and erectile dysfunction. It's crucial not to miss this diagnosis due to its impact on cardiovascular health and quality of life.
Testicular Cancer: Though rare, testicular cancer can lead to hypogonadism and thus the symptoms described. Early detection is crucial for effective treatment.

Klinefelter Syndrome: A genetic condition (XXY) that affects males, leading to hypogonadism and thus potentially all the mentioned symptoms. It's rare and typically diagnosed at a younger age but could be considered in undiagnosed adults.
Prader-Willi Syndrome: A rare genetic disorder that can lead to hypogonadism, muscle wasting, and other metabolic issues, potentially causing the described symptoms.
Myasthenia Gravis: An autoimmune disease that can cause muscle weakness and fatigue, though it less commonly presents with the full spectrum of symptoms described, including sexual dysfunction.