The best two screening blood tests for thyroid function are a free T3 (triiodothyronine) and free T4 (free thyroxin) both done by all labs. Another thyroid test, the TSH, should be done as follow up if the free T3 and free T4 are abnormal. Many doctors use a TSH for screening purposes, but it is not sufficiently sensitive and free T3 and free T4 are the tests of choice.
Low thyroid function can be treated with oral thyroid replacement medications. Armour Thyroid is the drug of choice. Oral T3 is available as Cytomel but is too short acting to be used. T3 cream can be compounded by a compounding pharmacist and is used in conjunction with Armour Thyroid.
Both free T3 and free T4 blood levels should be measured periodically for monitoring purposes.
Overproduction of thyroid hormone was formerly called Grave-s Disease. It is treated with oral radioactive iodine to inactivate the thyroid gland, or surgical removal. Thereafter thyroid replacement is given as if the thyroid had been absent to inactive in the first place. Hashimoto-s Thyroiditis is an autoimmune disease diagnosed with a reverse T3.