I understand that the majority of doctors consider the TSH test "the gold standard" for measuring thyroid function (examples stating that literally are here and here).
The reference range for what is a healthy TSH is under debate, but no matter what range is considered there tends to be a proportionately large difference between the top and bottom (eg 0.3 to 3.0 μIU/mL).
However I've never seen anything indicating over what range TSH varies over the course of a day, or a few days. It's also unclear as to how much error there tends to be with a test result, even if tested from the same sample.
How much fluctuation in a TSH result is normal over the course of a day?