Similar to in-office tests, at-home thyroid tests analyze your blood for levels of a myriad of biomarkers. Thyroid stimulating hormone (TSH) is the most common hormone measured by thyroid tests, although it’s not a hormone produced by the thyroid.
Instead TSH is produced by the pituitary gland and triggers the production of T3 and T4. TSH levels typically spike when T3 and T4 levels are low, indicating the body needs to produce more of these hormones (or vice versa). While the most affordable thyroid tests only look at the TSH, this biomarker does not provide a complete picture of your thyroid’s function.
Only looking at TSH assumes the body is operating normally and that no other issues are at play, which is a drastic oversimplification of the bodily process. In fact, a 2018 study2 found that screening for thyroid issues using a TSH test can actually miss hypothyroidism. Therefore, a normal TSH level does not ensure the thyroid is operating normally.
The most comprehensive view of your thyroid’s function includes direct tests of T3 and T4 levels, as well as thyroid antibodies and Reverse T3 or rT3. As previously mentioned, the first two are hormones directly produced by the thyroid. When testing, you can look at either your total levels of the hormones or the free levels, which are written at FT3 and FT4 or free T3 and T4. These “free” hormones are not bound to thyroxine-binding globulin, and are considered highly sensitive assays that are better to identify disease.
While most of the tests on our list look at FT3 and FT4, only a handful also keep on eye on rT3. When the body converts T4 to T3, it creates rT3 or an inactive form of T3 in specific ratios. Occasionally, the body may be blocked from making T3, instead overproducing rT3. This metabolite acts like a brake on the thyroid’s function, slowing down metabolism. As a result, it kickstarts many of the same symptoms as hypothyroidism, including weight gain. Both illness and chronic stress3 have been tied to high rT3 levels—which means it’s another important test to consider when making your selection.