The technology that went into producing the TSH test was impressive. They were detecting a truly tiny amount of TSH in a small (but massively larger) amount of blood.
Early TSH tests would be able to detect when TSH was low. They simply were not sensitive enough to identify the differences between the various degrees of "low".
The term "suppressed TSH" has come to mean one of a number of different (and inconsistent) things:
Any value of TSH below the lower reference interval. E.g. on a typical TSH test with an RI of 0.4 to 4.5, any value below 0.4.
Any value of TSH below the lowest measurable TSH on the specific assay used. A TSH test will be reported as something like < 0.01 - meaning lower than 0.01.
An arbitrary value of TSH below lower reference interval and the lowest measurable TSH.
Even if we assume that TSH does indeed drop as thyroid hormone levels rise, there is a problem.
A result that is below measurable TSH does not, cannot, show any difference between someone who has high thyroid hormone levels, but possibly still inside their reference intervals, and someone else who has thyroid hormone levels which are far, far higher - maybe double or more.
Yet many published papers treat all those with suppressed TSH as if they have significantly excess thyroid hormone. This wrongs to those who have acceptable thyroid hormone levels but are pushed to reduce thyroid hormone doses. And it also wrongs to those who really do have high thyroid hormone levels by diluting the pool and under-representing the potential severe symptoms they have or could develop.
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