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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?

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I wrote an answer about the circadian rhythm of TSH for a question over on Health.SE: How does one's TSH tend to vary over the course of a day. That answer also has some information on how this is different in people with hyperthyroidism and hypothyroidism.

The paper Physiological variations in thyroid hormones: physiological and pathophysiological considerations has a section on circadian and seasonal rhythm and a table with some values. Quoting from the paper:

There is a clear circadian variation in circulating TSH concentrations in animals and in humans. TSH concentrations are low during the daytime, increase in the evening, and peak shortly before sleep. Amounts decline slowly during sleep. Table 3 summarizes studies of mean, nadir, and peak serum

This is table 3:

TSH values in children and adults over thee course of a day

A chart is available in figure 2 of the paper Modulation of Endocrine Function by Sleep-Wake Homeostasis and Circadian Rhythmicity

As for test error, it's important to note that TSH itself is not used for or at least should not be used for diagnosis of Hypothyroidism and hyperthyroidism. T3 and especially T4 need to be measured.

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  • $\begingroup$ Heh, only now do I realise that it was actually you asking the question on Health.SE. Oh well, maybe it also helps someone here :-) $\endgroup$ – YviDe Dec 27 '15 at 9:37

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