Polycystic Ovary Syndrome (PCOS): This is the most likely diagnosis given the combination of amenorrhea, weight gain, and potential for hormonal imbalance. PCOS is a common endocrine disorder among women of reproductive age and is often associated with insulin resistance, which can contribute to weight gain and fatigue.

Hypothyroidism: This condition can cause fatigue, weight gain, and menstrual irregularities, including amenorrhea. It's a common endocrine disorder that can be easily screened for with thyroid function tests.
Hyperprolactinemia: Elevated prolactin levels can disrupt menstrual cycles, leading to amenorrhea, and are also associated with fatigue and potential weight changes.
Premature Ovarian Failure (POF): Also known as premature ovarian insufficiency, this condition leads to the loss of ovarian function before the age of 40, resulting in amenorrhea. Fatigue and weight changes can also occur due to the hormonal imbalance.

Pituitary Tumor: Although less common, a pituitary tumor (such as a prolactinoma) can cause hyperprolactinemia, leading to amenorrhea and other symptoms. Missing this diagnosis could lead to significant morbidity due to the potential for the tumor to affect other pituitary functions and cause neurological symptoms.
Thyroid Cancer: While rare, thyroid cancer can present with hypothyroidism symptoms, including fatigue, weight gain, and amenorrhea. Early detection is crucial for effective treatment.
Adrenal Insufficiency: This is a life-threatening condition that can cause fatigue, weight loss (though sometimes weight gain), and menstrual irregularities. It requires immediate medical attention.

Cushing's Syndrome: A rare endocrine disorder caused by excess cortisol, which can lead to weight gain, fatigue, and menstrual irregularities. It's essential to consider this diagnosis due to its significant impact on health if left untreated.
Congenital Adrenal Hyperplasia (CAH): A group of inherited disorders that affect the adrenal glands, leading to hormone imbalances. Some forms of CAH can present with amenorrhea, weight changes, and fatigue, although it's more commonly diagnosed in infancy or childhood.
Prader-Willi Syndrome: A rare genetic disorder that can cause a range of symptoms, including obesity, short stature, intellectual disability, and hormonal imbalances leading to amenorrhea. It's less likely but should be considered in the differential diagnosis, especially if other characteristic features are present.