What is likely to be the cause of an elevated ch50 but normal c3 & c4? my pcp tested me for lupus due to my chronic migraines & hypothyroidism...

Not useful: Reference ranges are set so that several percent of healthy folks fall outside at either end. Don't worry about an elevated CH50. Fishing expeditions like this are fine and your physician was being thorough, but order enough labs and you ALWAYS run into a few outside the range. Hope the headaches prove manageable and that you'll be compliant with your thyroid replacement.

Not much: Although an elevated CH50 suggests the presence of inflammation or infection, no diagnosis can be made from this finding alone Lupus is also diagnosed by clinical features supported by lab findings. Although hypothyroidism may be from an autoimmune disorder, I don't know why lupus is suspected absent any other symptoms. Migraine is not an autoimmune issue.