What are the differences between an endocrinologist's thyroid testing and a GP's?

A GP would likely only test TSH levels. And endo would test T3 T4 and I think they also test for your body’s ability to absorb thyroid hormone. (This would mean that you could produce a normal amount of thyroid hormone, but your body wasn’t processing it correctly. So your numbers would be normal or even slightly high, but your body would be acting hypothyroid. This condition is EXTREMELY rare so not likely, but an endo would likely check to rule it out.)

You can ASK for your GP to test these things and they likely will. The difference isn’t in the tests, it’s in the interpretation. There are new standards of what is normal in thyroid function and your GP may still be sticking to the old rules of thumb. A different GP may diagnose you with thyroid disease with the same numbers because they are going by the new numbers instead of the old.

Basically, what’s changed is that many people are more sensitive to thyroid hormone levels than others. So a smaller imbalance can cause hypothyroid than what was previously thought. So if you have the symptoms and your thyroid is only “slightly” low, you will likely get treated for thyroid disease.

Going to an endo would help rule out other endrocrine disorders that might cause similar symptoms. But if you ONLY want to test the thyroid, just ask the GP for more tests. Then have them explain the numbers. If your doc says your T3 or T4 levels are low or your TSH is high, but not off enough for a diagnosis, go to a different doctor with the same test results.

Nothing, necessarily, but it depends on which tests they run. A CBC alone isn’t enough. As @keobooks noted, T3, T4, and TSH are separate tests, but my GP runs those. I’ve have hypothyroid for nearly 30 years, and haven’t seen an endocrinologist once.