What could cause/explain significantly (4.5x norm rang)elevated plasma norepinephrine levels in blood test, but normal urinary norepinephrine levels?

Details? Was the urinary norepi a random or 24hr? Spot urines tend to not be as accurate as 24 hr. Lead toxicity is one of the things that can inhibit proper breakdown of norepinephrine.
Several things. Including daily doses of snri's -- medications used to treat depression. See your doctor; the evaluation can be complex.

Related Questions

What could cause/explain significantly elevated (4.5x the norm range)plasma norepinephrine in blood test, but normal urinary norepinephrine levels?

Normal variation. Plasma levels vary with rate of production and rate of excretion. Hormone levels vary a great deal in production by stress and by time of day and by medication effects. Rate of excretion depends on liver metabolism and clearance by the kidney. Kidney clearance may be normal but will eventually clear the hormone over time. High serum level means high production by some uncertain cause. Read more...
Exclude lab error. First: exclude lab error. Second: talk to your physician, who can give you detailed explanation. Read more...