Thyroid or pituitary?

With an extremely low TSH but normal T4, T3, and T7, what is the next step to see if there is more pituitary or thyroid dysfunction?

[AR-10]

It depends on the tests and the results.

If you are talking about a Free T4 test and a Free T3 test, and the results are high normal, then thyroid antibody testing would be suggested. Several different ones.

If the results are low normal, then pituitary testing would be warranted.

If it is a Total T4 test, and a Total T3 test, and a T7, then the tests results are bad and you need to have better tests run. These are old tests, and the "free" tests are better.