A patient presents with fatigue, weakness, weight loss, hypotension, and skin hyperpigmentation with elevated ACTH. What is the most likely diagnosis?

• A. Thyrotoxicosis
• B. Hypothyroidism
• C. Cushing's disease
• D. Addison's disease

Answer: (D)

Recognizing primary adrenal insufficiency (Addison's disease) explains this presentation. When the adrenal cortex fails to produce enough cortisol (and often aldosterone), the pituitary pushes out more ACTH in an attempt to stimulate the adrenals. That elevated ACTH also increases melanocyte-stimulating hormone activity, causing skin hyperpigmentation. Cortisol deficiency leads to fatigue, weakness, weight loss, and a tendency toward low blood pressure due to reduced vascular tone and, with aldosterone loss, salt wasting and volume depletion. The combination of fatigue, weight loss, hypotension, skin hyperpigmentation, and a high ACTH level is classic for Addison's disease.

Other conditions like thyroid disorders or Cushing's disease don't fit this pattern: thyroid problems can cause fatigue and weight change but not the hyperpigmentation driven by high ACTH, and Cushing's presents with features of cortisol excess rather than deficiency.