Patients, who are treated for hypothyroidism, sometimes are found to have "impaired T-4 to T-3 conversion". These are patients who for some reason, do not have the ability within their body, to adequately convert enough T-4 into the more needed T-3 hormone. Doctors do not always identify the problem that results in impaired conversion but it may at times be due to an inability of the liver to aid in the conversion process, due to liver disease.
There may also be co-morbid (co-existing) illness or disease of another kind in the body that hinders the conversion process of T-4 to T-3. Regardless of the cause that may or may not be found, these patients will need to have the low T-3 hormone replaced with thyroid hormone replacement therapy that includes both T-4 and T-3, rather than with a T-4 only thyroid hormone medication.
Most patients with hypothyroidism are prescribed a T-4 only brand of thyroid hormone medication and the needed T-3 hormone is converted from it, within the body successfully. In patients with the less common "impaired conversion" however, a T-4 only hormone medication will not supply them with adequate T-3 that is also needed in the body.
Unfortunately, some Doctors do not believe impaired conversion of T-4 to T-3 happens, except in extremely rare cases and this will result in less blood testing of treated hypothyroid patients, of their T-4 and T-3 levels because testing of these will be deemed unnecessary. These Doctors will believe the T-4 and T-3 tests are not necessary and they will only monitor these patients on T-4 only treatments for hypothyroidism using a TSH test only. The TSH or "Thyroid Stimulating Hormone" is not a thyroid hormone but is a pituitary hormone as discussed earlier that indicates how much thyroid hormone is available to the body in the blood stream. The problem however, with not testing both T-4 and T-3 levels in patients treated for hypothyroidism with a T-4 only medication, at least for the first one or two follow up retests, is that adequate T-4 in the blood will give a normal TSH reading even when the T-3 hormone level may be low.
In my opinion as a layperson and treated hypothyroid patient who has extensively researched this subject and corresponded with 1000s of patients for over five years, treated hypothyroid patients need to have their "Free T-4" and "Free T-3" levels (the "frees" are the tests of unbound, available hormone) tested, in addition to TSH, for at least the first two blood retests, in follow up on their T-4 only thyroid hormone replacement therapy. If these first follow up tests indicate that the hormone medication is resulting in adequate amounts of both T-4 and T-3 from proper conversion, TSH only testing afterward, would be sufficient for follow up retests, to monitor the hormone replacement therapy.
When thyroid hormone imbalances are detected however, other types of blood testing may be done to detect the cause of the thyroid hormone imbalance. Tests to detect "thyroid auto-antibodies", which are immune system cells that can mistakenly attack the thyroid and cause it to produce imbalanced amounts of hormones might also be tested for. If "thyroid autoimmunity", which is the most common cause of thyroid disorders in most industrialized countries, is not found, then blood tests of pituitary function may be ordered to test for central (brain-center) causes of thyroid hormone imbalance.
In more complicated cases, hypothyroidism might be secondary to another disease process in the body, in which case, extensive blood testing of all types may then be undertaken to determine the cause of the thyroid hormone imbalance. There are other diagnostic tests of thyroid function available, in addition to blood testing as well, such as imaging tests and thyroid tissue biopsies but these usually follow the blood tests that are first used to detect thyroid hormone imbalances.