Serum Free T4 Concentration: More Accurate and Reliable than TSH as an Indicator of Thyroid Gland Function

Author(s): Ashraf K Mena, Udaya M Kabadi

Objective: To compare the accuracy and reliability of Free T4 or TSH determination as the single initial screening test in assessment of thyroid function.

Methods: Retrospective review of 3,022 thyroid function tests (free T4 and TSH) performed during a 12 month period in a laboratory at a major academic medical center was conducted. A probable diagnostic category for each test was determined by a retrospective detailed review of medical records of subjects with discrepant serum free T4 and TSH concentrations.

Results: In 284 tests (9.4%) the appropriate diagnosis could not be arrived at if serum TSH concentration alone was used for interpretation of thyroid function. Alternatively, serum free T4 concentration alone was unable to provide accurate diagnosis in 132 (4.4%) subjects. Multivariate analysis determined that free T4 is more reliable than TSH as an initial test for the diagnosis of thyroid function as expressed by odds ratio (OR) 2.14 with confidence interval (CI) 1.96 - 2.34, p< 0.01. The sensitivity and specificity for diagnosis of thyroid disorder were superior for Serum free T4 concentration (91% and 94%) respectively when compared with serum TSH level (84% and 86%). However, the sensitivity and specificity for both laboratory tests assessed in conjunction improved further ( 98% for both) in comparison to either of the tests evaluated individually in diagnosis of thyroid disorder.

Conclusions: Free T4 may be more accurate and reliable than TSH as a single initial screening laboratory test in assessment of thyroid function. However, both performed simultaneously increases the reliability and accuracy in determination of thyroid functional state.