When the laboratory result for TSH is found to be decreased below the established reference interval, and the free levels of T4 or T3 are high, these findings support a diagnosis of primary hyperthyroidism. If, however, the TSH is high and the levels of FT4 or FT3 are also high or normal, these findings suggest a secondary cause for the hyperthyroidism. Finally, a decreased TSH with normal levels of free T4 and free T3 in the absence of overt clinical symptoms is interpreted as subclinical hyperthyroidism.
|TSH||Free T4||Free T3||Interpretation|
|Low||High or normal||High or normal||Primary hyperthyroidism|
|High||High||High or normal||Secondary hyperthyroidism|