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