Total thyroid hormone levels are not physiologically active in our bodies, but T3 and free thyroid hormone levels are. In this video, you’ll learn the nuts and bolts of how to interpret total thyroid hormone levels and why this figure does not provide an accurate reflection of thryoid function.
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[00:00:00] Most of the thyroid hormone that's present in circulation is bound to protein. Less than one-tenth of 1% of circulating thyroid hormone is free or active. When we measure total thyroid hormone, we're measuring both the bound thyroid hormone as well as the free thyroid hormone. Most assays for measuring total thyroid hormone are competitive immunoassays. In these assays, we have a radiolabeled thyroid hormone analog that's bound
[00:00:30] to an antibody-coated plate. The patient sample is added and the patient's thyroid hormone will displace the radiolabeled analogs. The sample is then washed and the displaced analog is rinsed away, leaving the bound analog to be measured. So, let's look at a few examples of how this works for our patients. So, let's say we have a patient with high thyroid hormone levels. This thyroid hormone is going to displace most of the bound analog, which is then going to be rinsed away leaving
[00:01:00] very low signal. Conversely, a patient with low thyroid hormone levels will only displace a few of the radiolabeled analogs. Those will be washed away, leaving a high signal. This signal is measured and from that result, we're able to calculate the amount of total thyroid hormone that was present in the original sample and that is the result that is reported out to you. T3 and T4 are both present in circulation and both can be measured using total thyroid hormone assays. However, T3
[00:01:30] is present at 5 to 10 fold lower levels than T4 so measuring this is more technically challenging. While total thyroid hormone assays are usually a fairly good reflection of the amount of free or active thyroid hormone present in the circulation, there are some situations where they are not accurate. Sometimes, the amount of bound thyroid hormone will be higher but the amount of free thyroid hormone will remain the same. So, in that case, you would have an increase in your total
[00:02:00] thyroid hormone level but your active thyroid hormone levels would remain unchanged. So, total thyroid hormone assays are rarely used as stand-alone measurements of thyroid function