Free T3 and reverse T3 are not the usual starting point for investigating fatigue. The American Thyroid Association recommends TSH as the initial thyroid-function test in most circumstances, with free T4 helping clarify the result.[1]

Working nights adds another important question: whether sleep is long enough, sufficiently uninterrupted and timed in a way that supports alertness.[2]

What do TSH and free T4 measure?

TSH is the signal that prompts the thyroid to make hormone. Free T4 measures the circulating hormone that is not bound to proteins. Their relationship helps identify different patterns of thyroid dysfunction.[1]

For example, high TSH with low free T4 can indicate an underactive thyroid caused by a problem in the gland. The interpretation changes when the pituitary, which makes TSH, is involved. Reading the two results together helps distinguish these patterns.[1]

Where do T3 and reverse T3 fit?

T3 testing can help investigate an overactive thyroid. It is usually less helpful for diagnosing an underactive thyroid because T3 may remain normal even when other thyroid tests are clearly abnormal. The association also notes that free T3 measurements are often unreliable and not typically helpful.[1]

Reverse T3 is an inactive form related to T3. In healthy people who are not hospitalized, measuring it does not help determine whether hypothyroidism is present, according to the association.[1]

Keep the work schedule in the picture

NIOSH describes insufficient sleep as sleep that is too short, repeatedly disrupted or poorly aligned with circadian rhythms. Those are distinct issues, and a thyroid panel cannot describe the schedule that produced them.[2]

A fatigue review can consider sleep, medicines, anemia and other possible contributors as well as thyroid disease. The choice of tests follows the history and examination.[3]

One practical detail matters before testing: biotin supplements can distort some thyroid results. Share the supplement list and follow the laboratory's preparation instructions rather than interpreting an unexpected result without that context.[1]