RE-ASSESSMENT OF THE TSH- FREE T4 RELATIONSHIP WITH ELECTROCHEMILUMINESCENCE ASSAY

Abstract

OBJECTIVES
In the initial assessment of thyroid function, Thyroid Stimulating Hormone (TSH) is often employed first followed by free T4 (fT4), since small changes in fT4 are accompanied by a much larger change in TSH from the time 3rd-generation TSH assays were introduced in 1990. This inverse TSH-fT4 log-linear relationship has been confirmed with many different assays. Roche Diagnostics, a popular automated immunoassay platform based on electrochemiluminescence, improved the sensitivity of their TSH assay from 0.01 to 0.005 mIU/L in late 2020. We studied the TSH-fT4 relationship with the new Roche assay on their Cobas e601 analyzer.

METHODOLOGY
All samples received from the primary health clinics with requests for both TSH and fT4 in 2021 were retrieved from the Laboratory Information System. Children (<18yo) and pregnant subjects were excluded. Where patients had repeat testing, only the earliest results were included to ensure that the thyroid tests were probably at their stable state. The assay measuring ranges are as follows: TSH 0.005- 100 mIU/L and fT4 0.5-100 pmol/L. Statistical analyses were performed using MedCalc Statistical Software v20.106 (Ostend, Belgium).

RESULTS
11369 subjects (Male = 2465, Female = 8904) aged 18-95 years (mean 48.8 ± 17.4) were studied. Excluding TSH values <0.005 and >100 mIU/L from the calculations (n=633), a significant inverse TSH-fT4 log-linear relationship was seen on regression analysis (log TSH = 1.104 - 0.0534 fT4; r = 0.38; p<0.001); TSH 0.006-99.9 mIU/L, fT4 0.88-100 pmol/L.

CONCLUSION
The assay is sensitive to changes in circulating fT4 and affirms the utility of TSH in thyroid function testing.