Although decreased thyroid hormone levels (hypothyroidism) can cause fatigue, it would be associated with other symptoms such as weight gain, constipation, dry skin etc. It does not typically affect sexual drive or ability to maintain an erection.

Increased prolactin levels, or hyperprolactinemia, can cause sexual dysfunction and decreased libido, but will also be accompanied by signs such as galactorrhea, menstrual irregularities in women, and vision changes due to pituitary adenoma, which are not present here.

Decreased insulin levels would lead to hyperglycemia, not noted on the patient's normal blood glucose levels. It might cause fatigue but erectile dysfunction is not a direct result.

While increased cortisol levels during periods of stress can result in fatigue, akin to our patient's symptoms, it does not typically directly affect sexual function.