Most thyroid screening stops at TSH, but TSH is the brain's signal to the thyroid, not the thyroid's actual output. The Complete Thyroid Panel measures TSH, Free T4, Free T3, Reverse T3, and Thyroid Peroxidase (TPO) antibodies. Free T3 is the active hormone that does the work in your cells, and the conversion from T4 does not always go smoothly, which is why someone can have a fine T4 and still feel hypothyroid. Reverse T3 acts as a brake: under stress or illness the body can convert T4 into reverse T3 instead of active T3, quietly dialing things down. It is rarely tested and can explain symptoms the standard numbers miss. TPO antibodies show whether the immune system is involved, which speaks to the cause rather than just the level. If you have been told your thyroid is normal but still feel exhausted, foggy, or cold, this is the fuller picture.