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But then why not just look at the t4 and t3 levels?

"Currently, measurement of the serum TSH concentration is the most reliable indicator of thyroid status at the tissue level. Studies of mild (subclinical) thyroid hormone excess or deficiency (abnormal TSH/normal range FT4 and FT3) find abnormalities in markers of thyroid hormone action in a variety of tissues (heart, brain, bone, liver and kidney). These abnormalities typically reverse when treatment to normalize serum TSH is initiated" [1]

The reference has loads more detail...

[1] https://www.aacc.org/-/media/Practice-Guidelines/Thyroid-Dis...

I wasn't arguing that, I was suggesting that if it wasn't clear why the tsh level is high or low, to add on a t4 level check. Which is what is currently done, at least in the US.

Well, there's a common condition called subclinical hypothyroidism which may be asymptomatic and harmless, or it may indicate thyroid disease and be accompanied by various non-specific symptoms. It's defined by slightly elevated TSH but normal T4 test results. Anybody with that condition would not be helped by what you are proposing.

In general, it seems to me that you will get some false negatives when using a less sensitive test result to qualify a more sensitive test result.

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