One of the reasons why three tests are used is because none of them are very accurate. I have seen many persons who were told they were in the "normal" range, but when they test themselves with the accurate temperature test below, they were found to be very hypothyroid (low in activity).

Dr. Broda Barnes, a physician way ahead of her time, found over 50 years ago that the body basal temperature was a good indicator of thyroid activity, in particular, the body temperature on arising from sleep. She wrote a book about thyroid activity and the various ills and problems arising from low (hypo) thyroid activity.

I am indebted to Dr. Roy Kupsinel of Orlando Florida for introducing me to Dr. Barnes work, and for various other helps he gave me while I was in practice there, and gradually learning about Alternate and Orthomolecular Therapies..

Testing Thyroid Activity
At night, shake down a thermometer - be sure that it is shaken down and below 95 degrees. Next morning, on awakening, put the thermometer under your arm with the bulb in the armpit with no clothing between it and the armpit. Leave it there for 10 minutes (use snooze alarm if you wake up to an alarm). Just drowse for that time lying still.

After 10 minutes, take the thermometer out, and read it, writing down the result right away. (On waking, most people don't think clearly and might forget the reading). This is known as your Early AM Basal Temperature, and the "normal" should be between 97.8 and 98.2. This reading taken by armpit is somewhat lower and somewhat more accurate than that taken by mouth. If you have a low-grade infection this may read higher than your "normal", therefore if it's in that range above, you should repeat the above procedure every other day for a week or so. If a menstruating female, also do it on the 2nd and 3rd day of your period.

If lower than the above range, you are probably hypothyroid, and if higher, then you are probably hyperthyroid, or you may have an infection somewhere. (Hypo means low in docterese, and hyper means high.)
Just some of the symptoms of hypothyroidism can be: depression, low energy, fatigue, many infections, chronic headaches, circulatory problems, chronic skin problems, poor memory and/or concentration, eczema, psoriasis, irregular menstrual periods, neuroticism, irritability, hair loss, and lots more.

A serious problem that many who are hypothyroid suffer from is obesity. Because the thyroid governs metabolism rate, and if it is hypo in activity, the body stores energy in the form of fat. A hyperthyroid person is usually always fairly thin no matter what they eat. (We all hate those lucky ones!)
The book by Dr. Broda Barnes is excellent.

To treat hypothyroidism, you can try to find an MD, or a DO, who will write you a prescription for Armour Natural, or Cytomel. Synthroid is what is mostly prescribed, but it has been linked to osteoporosis. There is also Lugol which has been found useful in conjunction with thyroid. Or, you can go to a good health food store and get a "glandular" that contains thyroid (along with other glandulars usually). Start with two glandulars/day for a week, and use the above temperature test to determine if that is correct for you.