For a 32-year-old man with weight loss, dizziness especially on sudden posture change, and hyperpigmentation, what is the most likely biochemical finding?

The most likely biochemical finding in this scenario is an elevated plasma adrenocorticotropic hormone (ACTH) level. The combination of symptoms, including weight loss, dizziness upon standing (indicative of possible adrenal insufficiency), and hyperpigmentation, points towards primary adrenal insufficiency, commonly associated with conditions like Addison's disease.

In primary adrenal insufficiency, the adrenal glands do not produce enough corticosteroids, particularly cortisol. This lack of cortisol results in a compensatory increase in ACTH from the pituitary gland. Therefore, in this case, a plasma ACTH level of 95 pmol/L suggests an inappropriate high level due to inadequate cortisol production from the adrenal cortex.

Other options provided do not align as closely with the symptoms and the likely diagnosis. For example, while low fasting plasma glucose could reflect adrenal insufficiency by indicating inadequate gluconeogenesis, the hyperpigmentation and standing dizziness provide stronger evidence for the ACTH elevation. Similarly, elevated serum sodium would not be typical in primary adrenal insufficiency, as sodium levels often decrease due to the deficiency of aldosterone. The 24-hour urinary free cortisol measurement being within normal limits at 60 nmol does not fit the diagnosis of primary adrenal insufficiency, where cortisol levels would generally be