--they are designed to measure different aspects of thyroid function. The thyroid "panel", though, might include T4, T3 and antibody assays (see below--check to see it if does).

The TSH test is for thyroid stimulating hormone--this is produced by the pituitary gland to stimulate the thyroid to produce more thyroxine if the feedback loop indicates the amount of hormone is insufficient for the body's metabolic needs. Do be aware that the different labs have different ranges for this now, and not all follow the American Academy of Endocrinology 2007 recommendation that the ranges for normal TSH be lowered in response to undertreatment of "subclinical" hypothyroidism (from .5 to 5.0 to .3 to 3.0). Even so, the TSH level is a grosser measure than the T3/T4, actual measures of thyroid hormone, would be.

The "free" tests are often more accurate than the others as a regular T4 (or T3) tests can be affected by the amount of protein (particularly albumin and thyroid binding globulin) in the blood, as thyroxine is a protein bound hormone. T3 tests for Triiodothyronine, which is produced in much smaller amounts than thyroxine, but has a greater affect, by weight, than thyroxine on metabolism. Under normal conditions, much of the T4 produced is eventually converted to T3. There are certain conditions in which enough T4 is produced but not enough is converted to T3--often this involves autoantibodies. These can be tested for as well--there are anti-TPO and anti-thyroglobulin assays.