Addison's Disease: This condition, characterized by the inadequate production of adrenal hormones, can lead to a variety of symptoms including abdominal pain, nausea, joint pain, lethargy, irritability, unintentional weight loss, and changes in skin pigmentation such as darkening of the skin in the creases of the arms and darkening of the gums. The hormonal imbalance affects multiple bodily systems, explaining the wide range of symptoms.

Hemochromatosis: This genetic disorder leads to excessive iron accumulation in the body, causing symptoms like joint pain, lethargy, and skin darkening. Although less directly linked to gum darkening, the systemic effects of iron overload could contribute to a wide range of symptoms.
Wilson's Disease: An inherited disorder that causes copper to accumulate in the body, particularly in the liver, brain, and other vital organs. Symptoms can include abdominal pain, joint pain, lethargy, and changes in skin pigmentation. While less common, it could explain the breadth of symptoms.
Diabetes Mellitus: Especially if uncontrolled, diabetes can lead to a variety of systemic symptoms including weight loss, lethargy, and potentially changes in skin and gum health due to poor glucose control and associated vascular changes.

Pheochromocytoma: A rare tumor of the adrenal gland that leads to excessive production of adrenaline, causing episodes of hypertension, sweating, headache, and abdominal pain. Although less likely, missing this diagnosis could be fatal due to the risk of hypertensive crises.
Adrenal Insufficiency (Acute): Similar to Addison's disease but represents an acute deficiency of adrenal hormones, often due to sudden loss of adrenal function. This condition is life-threatening and requires immediate medical attention.
Malignancy: Certain cancers, especially those affecting the adrenal glands, liver, or pancreas, could potentially cause a wide range of systemic symptoms including weight loss, lethargy, and changes in skin pigmentation.

Alkaptonuria: A rare genetic disorder affecting the body's ability to break down certain amino acids, leading to the accumulation of homogentisic acid. This can cause darkening of the skin and other tissues (ochronosis) and potentially joint and other systemic symptoms.
Porphyrias: A group of disorders that result from a buildup of porphyrins, which are compounds required for hemoglobin production. Certain types can cause abdominal pain, neurological symptoms, and changes in skin pigmentation.
Congenital Adrenal Hyperplasia (CAH): A group of inherited disorders that affect the adrenal glands, leading to hormone imbalances. While typically diagnosed in infancy or childhood, late-onset forms can present in adults with non-specific symptoms including weight loss, lethargy, and changes in skin pigmentation.