Which blood test is best depends on each individual case and depends on factors such as whether the case involves diagnosing thyroid hormone imbalance or if it is follow-up on treatment to correct a problem.
TSH the Most Sensitive Test
Thyroid Stimulating Hormone (TSH) is a pituitary gland hormone, which is sent from the brain-center or what might be referred to as the “central command post” that monitors and regulates the amount of thyroid hormone released from the thyroid gland. It increases when the thyroid needs to produce more hormone and it decreases when it needs to produce less but in a non-diseased thyroid gland, these fluctuations stay within normal values. When hypothyroidism (under-active thyroid) or hyperthyroidism (overactive thyroid) occurs, TSH will begin to fall outside of the normal values range.
Because of how finely-tuned TSH keeps the thyroid gland, it will change its level in the body with even to most subtle changes in thyroid function. If for example the thyroid becomes less able to provide adequate hormone in the body due to damage in the gland from a disease process affecting it, TSH will increase in level before thyroid hormones decrease in levels. It will continue to do so even when the thyroid becomes unable to supply normal amounts. During its attempt to keep the thyroid going at normal speed, the TSH hormone will be what first becomes abnormal on blood lab tests. This makes TSH the single best and earliest indicator of developing thyroid hormone imbalance that is available.
Free T4 and Free T3
The T3 and T4 thyroid hormone levels can be tested as the “total” levels or the “free” levels. Many thyroid specialists and endocrinologists believe that the free levels are the better blood tests. This is due to the fact that patients, who take cortisol steroid treatments, estrogen therapy or have certain medical conditions such as liver disease or pregnancy, can experience a change in the level of a thyroid protein called TBG (thyroxine-binding globulin). Much of the T3 is bound in the blood by this protein (total level) with the free, unbound T3 that is left over, being more active in regulating bodily metabolism.
Certain types of scenarios can change the free-circulating level of remaining T3 hormone in the body that is available including cases of thyroid disease. Depending on each particular case, this can result in Total T3 testing normal, while the Free T3 will test outside of normal values, better detecting thyroid hormone imbalances in some cases.
These two thyroid hormones (T3 and T4) are tested both to detect thyroid hormone imbalances and to monitor replacement hormone therapies. They are also sometimes tested to monitor thyroid hormone levels in hyperthyroid patients who have received treatment to slow thyroid hormone production or after they have had their thyroid glands removed as a treatment procedure. Often, TSH is added to the Free T3 and Free T4 for a more thorough evaluation of patients because TSH alone will not always detect all types of thyroid hormone disorders, such as those caused by a problem within the thyroid-regulating brain glands rather than a problem within the thyroid itself.
Thyroid Panels
These groupings of tests, are ordered to better evaluate patients being diagnosed and will include both TSH and thyroid hormone levels (T3 and/or T4). A thyroid panel is often a better diagnostic tool than is TSH alone or a single thyroid hormone level. This is because of factors including those discussed in the previous subheadings in which a thyroid hormone problem can be either primary (within the gland) or secondary (outside of the gland).
This can also be true of thyroid hormone therapies in which TSH alone is not accurate in monitoring treatment due to regulating brain gland problems or due to a problem with only one thyroid hormone level rather than both. The need for varied or for more thorough types of thyroid blood testing can occur with “Low T3 Syndromes” or in cases of “impaired conversion of T4 into T3” that is experienced by some thyroid patients taking T4 only replacement hormone medications.
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