The thyroid makes two hormones: T4 (thyroxine) and T3 (triiodothyronine). The production of thyroid hormones is in response to TSH (thyroid stimulating hormone), which is produced by the anterior pituitary gland in the brain. Since there are several hormones involved in thyroid function, it makes sense that these should be tested for as a part of your thyroid labwork.

No thyroid hormone is more important than the other. Each thyroid hormone has a role to play to keep the thyroid functioning optimally; however, there is a thyroid hormone that does not get the respect it deserves.

When I review thyroid lab test results for clients seeking a second opinion, I find that T4 is more likely to be tested than T3 (in addition to TSH of course).

At least 90% of thyroid hormone production is T4. After T4 is released by the thyroid, ideally 60% is converted to T3. While T4 is more abundant, T3 is more potent. A common problem with thyroid disorders is that not even T4 is converted to T3; rather, it is converted to reverse T3 (rT3) that offers no health benefit. Checking rT3 levels can therefore be helpful for assessing thyroid hormone conversion and consequently managing thyroid disorders.