When the pituitary senses T3 and/or T4
levels getting too low, it puts out more TSH and vice versa. Because TSH is a pituitary
hormone, it can be affected by many issues besides T3 and T4 levels (like pituitary and hypothalmus issues, among others). Therefore, TSH is not the most reliable indicator of thyroid status. FT3 and FT4 tests measure the levels of T3 and T4 actually available to your body. FT3 and FT4 are the key to diagnosing and treating thyroid conditions.
Stella's right..