Step I TSH vs t4/t3

[nurmd01]

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The first line throid function test to assess throid status is TSH. Second line is T4. Why is this?? I mean the way I understand it is that if something is functioning we look at its product (t4/t3) not what stimulates it (tsh).

 

[seminoma]

The first line throid function test to assess throid status is TSH. Second line is T4. Why is this?? I mean the way I understand it is that if something is functioning we look at its product (t4/t3) not what stimulates it (tsh).

TSH is cheaper. T4 can be normal in subclinical hypo/hyper thyroid. Low T4 tells you hypothyroid, but it doesn't tell you if it's central or peripheral.