Let me give you an example...
When it comes to thyroid function, most doctors only order TSH. This is 1 of at least 5 markers that should be looked at.
All TSH tells us is if your brain is telling your thyroid to make thyroid hormone. Literally, that is it!
It doesn't tell us how much thyroid hormone is being made, how much of that is being converted to the active form, if you have issues getting the active thyroid hormone into the cell or if you have any thyroid antibodies that may indicate an autoimmune issue with your thyroid.
Not only that, but the "normal" range for TSH is usually 0.5 - 4.0. So, if your results came back at 3, the doctor would say that everything was normal.
From a functional side, we want TSH between 1 and 2. From this perspective, a TSH of 3 would indicate hypothyroidism.