TSH—why and how we measure it

In this video, you'll learn why TSH is ideal for monitoring thyroid disease and the method by which TSH is commonly measured–the so-called sandwich technique.

Tracy Tylee, MD
Tracy Tylee, MD
26th Jul 2017 • 2m read

This video was taken from our Thyroid Essentials course. In it, you’ll learn why TSH is ideal for monitoring thyroid disease. You’ll also learn about the method by which TSH is commonly measured–the so-called sandwich technique.

Enjoy, and don’t forget to check out the course!

Video Transcript

[00:00:00] Thyroid stimulating hormone or TSH is the best assay for assessing thyroid function, for a given individual. It's very sensitive to any changes in thyroid hormone levels. For instance, you can see here if your thyroid hormone level increases by 50%, your TSH level is actually going to decrease tenfold. So, small changes in thyroid hormone levels can lead to large changes in TSH. Current highly sensitive TSH assays

[00:00:30] utilize an immunometric assay, known as the sandwich assay. In this case, the captured antibody is bound to a plate, which binds to the patient's TSH. A second labeled antibody is added to the mix, which binds to a different portion of the TSH molecule creating a TSH sandwich. After you wash off the unbound antibody, another substrate is added, which reacts with the labeled antibody and emits a signal that can be measured, reflecting the amount of TSH in the patient's

[00:01:00] blood. TSH is a very good method for monitoring thyroid function, for a given individual, as it has very low intraindividual variability. So, what this means is you can measure the patient's TSH one month and then six months later, measure their levels again and they should be relatively similar. If there has been a significant change in their TSH levels over that time, even if still within the normal range, that may be an indication that that patient is developing thyroid dysfunction and should be followed more closely. There are some limitations

[00:01:30] to the TSH, however. TSH tends to lag behind changes in thyroid hormone. For instance, if a patient's thyroid hormone levels are dropping quickly following thyroid surgery for instance, your T4 levels will be in the abnormal range but the TSH has not yet started to increase. Similarly, if a patient is started on thyroid hormone replacement and their levels are returning back towards normal, the TSH takes a little bit longer to come back into the normal range. So, in times of rapid changes in thyroid hormone levels, the TSH may

[00:02:00] not be an accurate reflection of thyroid function. It is best used to monitor thyroid function in a steady state. There are also some variations in the normal range for TSH. Pregnant women tend to have a lower average TSH, whereas elderly patients tend to have a higher average TSH. Even if these values fall out of the normal range, they are normal for these individuals. So, it's important to consider your patient's background when interpreting the TSH results.