The TFTs, (TSH, T4, T3, FT4, FT3, Tg,) are useful only if you understand their limitations. There are four areas that are problematic.
The first is that until very recently there has been no attempt at a unified standard. Each lab sets their own reference range either from the vendors calibration kit or from establishing a reference range based on a minimum of 120 highly qualified subjects. This means that if you sent a single serum sample to 10 different labs you would get 10 different results with 10 different reference ranges. If you ran a single serum sample on 20 different vendors equipment you would get 20 different results with 20 different reference ranges. You also have the problem that the different chemicals used by the different vendors will react differently to some of the TSH isoforms. In 2007 The IFCC, International Federation of Clinical Chemists, established a committee to 'harmonize' the TFTs so the results from various labs and vendors will be within plus or minus 5%. This is still a work in progress. The FT3 test had a variance of up to 60% so that is going to take some work.
The second problem has to do with the reference ranges, most notably with the TSH. In 2002 the NACB, National Academy of Clinical Biochemists, said that in the future the top of the TSH should be 2.5. In 2002 AACE, American Association of Clinical Endocrinologists, suggested a RR of 0.3-3.0 because the TPO and TG AB tests start showing results around 3.0. That started the TSH reference range wars which Mary Shomon has written about. Today in the US most RRs run from 0.1-0.5 on the bottom and 4.0-6.0 on the top.
The third, and perhaps the most critical, problem is the 'low index of individuality'. Each of us has our own individual reference range which is actually fairly narrow, about plus or minus 1.0 from our own mean. Also, all of us have slightly different set points in our bodies for TSH, T4 and T3. We do not all share the exact same set point, it varies for each individual. This means the TFTs cannot tell you where you are in regards to your own personal reference range and set point. They can only tell you how you relate to the broad range of a group of people. This is very a important concept to understand. The reference range is the broad side of a barn. Somewhere on that broad side is a door large enough to drive a tractor in. The door is your own personal reference range. Where it is on the side of the barn varies from barn to barn. The TFTs only know the broad side of the barn, they have no idea where the door is.
The laboratory reference range is not the patient's reference range.
As Dr. David Derry said, "The TSH had a ring of scientific rigor for those who have a smattering of knowledge about thyroid metabolism. It was part of the pituitary feed back mechanism for monitoring the output of the thyroid gland. There is no doubt that it does accomplish this job. But unfortunately the TSH value has no clinical correlation except at absolute extremes with the clinical signs or symptoms of the patient."
Lastly, most allopathic doctors treat the TFTs as absolute and infallible, if your results are in the reference range it can't be a thyroid problem, period. Unfortunately, there are several problems that the TFTs can miss completely such as conversion problems, thyroid hormone resistance or the early stages of Hashimotos. This results in an inferior standard of treatment that leaves many of us sick, or makes us sicker, or destroys our relationships with our loved ones, or destroys our careers, or destroys our lives.
There is more to it than that but this is a simplified outline. I have included a link to a good presentation, about 18 minutes. You want "TSH Reference Range Redefined" by Dr. Lee. This will not work on a Mac, only on a PC. PR