When the laboratory result for TSH is elevated above the established reference interval and the free levels of T4 or T3 are low, these findings support a diagnosis of primary hypothyroidism. However, if the TSH is low and the free T4 (FT4) or free T3 (FT3) levels are also low or normal, these findings suggest a secondary cause for hypothyroidism. Finally, an elevated TSH with normal levels of FT4 and FT3 in the absence of overt clinical symptoms is interpreted as subclinical hypothyroidism.
Table 4 provides a summary of the laboratory findings and the corresponding hypothyroid state.
Table 4. Hypothyroidism Interpretation.| 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 |
