My doctor ran blood work to check for thyroid problems. The first test she ran, she said, was a screening test. The report calls it "TSH, 3rd Generation". Mine was below the range at 0.18 while the reference range given was 0.40-4.50 mIU/L. At this point she said she suspected hyperthryoidism which is the opposite of what's usually common and she prescribed another blood work order to confirm for a "T4, Free". The range given was 0.79-2.35 ng/dL and mine was within the range at 1.28.
By the second test, it seems as if my thyroid levels are fine... so what does the low value of the first test ( the "TSH, 3rd Generation") mean?
The first is a lab error. Yes, they do occur, in fact, they are not uncommon. So, if a patient has a lab finding which does not seem to fit, it should be repeated, just to be sure it is correct.
However, if the testing is correct, the whether high or low, an abnormal TSH indicates an excess or deficiency in the amount of thyroid hormone available to the body, but it does not indicate the reason why. An abnormal TSH test result is usually followed by additional testing to investigate the cause of the increase or decrease.
When the TSH is low and the T4 is normal, it usually indicates a very mild (or subclinical) hyperthyroidism. It has not yet reached the severity that needs treatment yet.
But, again, any time the lab results are a little confusing, they should be repeated.