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Hypothyroidism: Laboratory Findings

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

High

Normal

Normal

Subclinical hypothyroidism