Why are there such different 'normal' ranges of IGF-1?

[imom]

Looking at my BW, I see the lab gave me a reading of 141 ng/ml and gave the normal range for my age group (41-50) as 160-318. Wikipedia gives a normal range from 71-223 for a 45yo. Why such a difference? According to one range, I'm deficient, and on the other range I am almost average.

[thisAngelBites]

Reference ranges on lab tests are done by drawing bloods from an apparently healthy group of people (people without any symptoms for illnesses that might reflect an illness related to the test range being established) and then extrapolating from that what the range of variance is among healthy people in general.

But people who appear to be healthy can have all sorts of things going on that they don't have any knowledge or symptoms of, and there can be huge differences in lifestyle that impact the values individuals in the reference group that are not accounted for (diet, for example).

I had a friend who had a high TSH of 3.2 (well, I would consider it high for me, as I would be sick at a TSH that high) but had absolutely no signs of hypothyroidism. About two months later, she developed lots of signs of being hypo, and went and was tested again and this time TSH was 3.4. So she could have contributed to a reference group at the time of the first TSH test, when in actuality she was on the doorstep of hypothyroidism without knowing it.

Also, even without these difficulties, average ranges do not even aim to say anything about what optimal ranges might be (although many physicians treat them as though they do). So most of the time you have to take reference ranges with a grain of salt and take your symptoms into strong consideration as well as test results in order to get a broader picture of health.