Abnormal OGTT Results - The many causes
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As often happens, when looking for something a bit more specific, I came across this image at right, which led me to this webpage: Laboratory Testing for Diabetes Mellitus.
Curve 1 is a normal OGTT, Curve 6 is a diabetic. The other causes are listed:
- Hypercorticism - curve 5.
- Acromegaly - between curves 4 and 5
- Hyperthyroidism - curve 4.
- Pheochromacytoma (or "emotional hyperglycemia") - between curves 4 and 5
Pathologic conditions causing flat or depressed glucose tolerance results:
- Insulinoma - Curve 2 or even more depressed
- Intestinal malabsorption - curve 2 or even less of an increase
- Low renal Tm for glucose reabsorption - may be normal curve 1 or may be depressed as in curve 3
- Hypothyroidism - curve 2.
The cite contains a descriptive of each condition. I'm mostly presenting this as an FYI. One thing I was surprised by was the hypothyroid which delays the glucose absorption for a later peak with albeit suppressed glucose levels. Couple hypothyroid with impaired pancreatic function and what do you get? I don't know the answer to this, but can envision that there are a number of people in the community who might have "glucose intolerance" due to a mismatch between absorption, signaling and secretion.
Comments
I don't know the other conditions (except hyperthyroidism), but definitely the hypothyroid curve is not correct. See, for example (in this study none of the subjects was receiving any treatment
or had a family history of type 2 diabetes):
http://eje-online.org/content/160/5/785.full.pdf+html
Is there any reason to believe that a rise in the prevalence of these non classic cases is related to the diabetes epidemic?
I think it would be best that all OGTT's include insulin levels as well.
That is a very interesting paper on thyroid and IR, as this is true peripheral IR as we think of it -- e.g. impaired glucose clearance due to reduced transporters/rate.
It's not practical, but a radiolabel OGTT w/insulin levels would be far better to tease out the underlying pathologies.
My OGTT confirmed that a high carb diet is not the best thing for me. By moderating my carb input, I've completely eliminated bouts of "weak and dizzy". Where is Nigel when you need him? (We are all different.)
Here's my point. Studies usually report averages. Statistical "significance" only predicts that the next similar study's average wouldn't likely differ by much. I tire of the "X diet is the only one for everybody" advice found in both the blogosphere and doctor's offices.
You appear to have isolated impaired fasting glucose (IFG). Sigh, I hate when drama sidetracks me because I have an interesting paper to get around to blogging on regarding the incidence if IFG and IGT. As Karen states, you'd be diagnosed diabetic with your IFG, but it sounds like your postprandial insulin response is normal. The elevated FBG is usually indicative of hepatic insulin resistance -- failure to suppress gluconeogenesis adequately -- metformin may be in order here. Interestingly, IFG does not appear to be as predictive of problems as IGT, so that may be relatively good news for you. How's your HbA1c? Also here I'd love to know NEFA levels but they aren't routinely measured.
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