Age-related hyperpigmentation: This is a common condition in elderly patients, characterized by an increase in melanin production and accumulation over time, leading to generalized hyperpigmentation. The normal morning cortisol level helps to rule out other endocrine causes.

Vitamin B12 deficiency: This condition can cause hyperpigmentation, particularly in the skin and mucous membranes, due to the role of vitamin B12 in melanin synthesis. Other symptoms such as fatigue, weakness, and neurological changes may also be present.
Iron deficiency: Similar to vitamin B12 deficiency, iron deficiency can lead to hyperpigmentation, as iron plays a role in the synthesis of melanin. Other symptoms such as fatigue, weakness, and pallor may also be present.
Diabetes mellitus: Some patients with diabetes may develop hyperpigmentation, particularly in the skin folds and creases, due to insulin resistance and inflammation.

Addison's disease: Although the morning cortisol level is normal, Addison's disease should be considered, as it can cause hyperpigmentation due to the increased production of adrenocorticotropic hormone (ACTH). Other symptoms such as fatigue, weight loss, and hypotension may also be present.
Malignancy: Certain types of cancer, such as melanoma or lymphoma, can cause hyperpigmentation. It is essential to rule out these conditions, as they can be life-threatening if left untreated.

Hemochromatosis: This is a genetic disorder characterized by excessive iron accumulation, which can lead to hyperpigmentation, as well as other symptoms such as liver disease and heart problems.
Porphyria cutanea tarda: This is a rare genetic disorder that affects the production of heme, leading to hyperpigmentation, as well as other symptoms such as blistering and scarring of the skin.
Nelson's syndrome: This is a rare condition that occurs in patients who have undergone bilateral adrenalectomy for Cushing's disease, leading to hyperpigmentation due to the increased production of ACTH.