Someone asked me to give further info about my claim that HbA1c tests cannot be relied upon if you are being treated with levothyroxine. I can't find that request now so I'm starting a new thread.
The clearest statement is on the American Thyroid Association website.
WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study suggests that hypothyroidism may be falsely increasing the levels of the HBA1C test. While thyroid hormone therapy decreases the HBA1C test results, suggesting an improvement of blood sugar control, actual measurements of fasting blood sugars and overall glucose tolerance were unchanged on thyroid hormone therapy. This may lead to errors in diagnosing pre diabetes and diabetes in patients with hypothyroidism. This is important for both physicians and patients to know.
Although the study commented on mis-diagnosis of pre-diabetes and diabetes in people with overt untreated hypothyroidism but healthy underlying blood glucose levels (upward distortion of HBA1c) it explains that the distortion arises because of the effect of hypothyroidism and levo on the turnover of red blood cells. The HBA1c tests the amount of glycogen attached to old blood cells. The basis of the test just doesn't work with either hypothyroidism or treated hypothyroidism and so it not accurate full stop.
I have excellent HBA1C of 33. My fasting glucose is or was around 6.2 which is pre-diabetic.
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Pearlteapot
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The issue is not with treated hypothyroidism ie those on sufficient levo, but with untreated hypo. Also b12 & iron deficiency, among others.
"Thus in hypothyroidism there is a very high false positive rate for the diagnosis of dyglycemia, if HbA1c alone is used as the diagnostic test."
"The glycated hemoglobin represents the fraction of hemoglobin that undergoes non-enzymatic glycation over the circulatory life span of the erythrocytes (usually 120 days) (3,4). It not only depends on the ambient level of glycemia over the preceeding 2-3 months (5), but also on the average period of exposure of the circulatory red blood cells (RBCs) to this glycemia i.e on the erythrocyte turnover in circulation (3,5). Conditions which are associated with a low RBC turnover, with a predominance of older cells (and a paucity of younger RBCs and reticulocytes) in circulation are associated with a falsely elevated HbA1c. Proven examples include iron (6,7) and vitamin B12 deficiency (6), and renal failure (8)."
I don't think it is just limited to the false positives for dyglycemia. It also means false negatives.
If you test before and after starting thryoid treatment over a short time span you might have the effect that the false positive is unravelled and the reduction in HBA1c reflects the underlying glucose level that was present at the date of the false positive.
You might, but that is not what it says. It says that treatment with levo reduces HBA1c without reflecting underlying glucose levels.
What if underlying glucose is rising over time while being treated with levo - a common enough scenario. HBA1c does not reflect underlying glucose levels.
There are a great many comments about the effects of hypothyroidism on medications and procedures.
It should be made clear in these papers, PILs etc that these comments relate to untreated hypothyroidism. There is a tendency, even on this forum, for people to claim that they are hypothyroid even when they are being successfully and optimally treated with exogenous hormones
That's rather a broad assertion. Some papers refer to untreated hypothyroidism. This one comments on both untreated and treated hypothyroidism.
Most people on this forum are not optimally treated - that's why they are here. For those that are optimally treated they have a hypothyroid diagnosis because they no longer produce sufficient thyroid hormones endogenously. They are being treated with greater and lesser degrees of success.
Why do you object to people calling themselves hypothyroid with reference to their diagnosis?
I don’t object. It simply muddies the water when looking at drug interactions and conditions which are caused by hypothyroidism. I have just been prescribed a new medication which is contraindicated in hypothyroidism. I have no thyroid but as I’m optimally medicated it isn’t an issue.
Crikey something else we can’t rely on. Good game for Gps as each ‘positive’ test gives them more money in their coffers and ‘false’ negative for the patient ‘Heyho’.
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