The concentration of thyroid hormones (T3 and T4) in the blood regulates the pituitary release of TSH; when T3 and T4 concentrations are low, the production of TSH is increased, and, conversely, when T3 and T4 concentrations are high, TSH production is decreased. This is an example of a negative feedback loop. Any inappropriateness of measured values, for instance a low-normal TSH together with a low-normal T4 may signal tertiary (central) disease and a TSH to TRH pathology. Elevated reverse T3 (RT3) together with low-normal TSH and low-normal T3, T4 values, which is regarded as indicative for euthyroid sick syndrome, may also have to be investigated for chronic subacute thyroiditis (SAT) with output of subpotent hormones. Absence of antibodies in patients with diagnoses of an autoimmune thyroid in their past would always be suspicious for development to SAT even in the presence of a normal TSH because there is no known recovery from autoimmunity.