The researchers studied how a hormone called TRH affects the thyroid gland. They gave TRH to people with normal, underactive, and overactive thyroid glands. In normal people, TRH increased levels of TSH and T3, but not T4. In underactive thyroid patients, T3 and T4 did not increase in response to TRH. They found that TRH can help diagnose different types of thyroid problems based on TSH levels.