The page below is a sample from the LabCE course Laboratory Assessment of Thyroid Function. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Hyperthyroidism: Laboratory Findings

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

FT4

FT3

Interpretation

Low

High or normal

High or normal

Primary hyperthyroidism

High

High

High or normal

Secondary hyperthyroidism

Low

Normal

Normal

Subclinical hyperthyroidism