Estimation vs measurement of serum free T4 in the pregnant state

TOPIC: Free T4 measurements during pregnancy

Title: Tandem mass spectrometry improves the accuracy of free T4 measurements during pregnancy.

Authors: Kahric-Janicic N, Soldin SJ, Soldin OP, West T, Gu J, & Jonklaas J.

Reference: Thyroid 17: 303-311, 2007

Summary

Objective

The aim of the study was to compare a new tandem mass spectrometric (LC/MS/MS) method for measuring free T4 in pregnancy with free T4 measurements made by equilibrium dialysis (ED) and an immunoassay method.

Subjects & Methods

Ninety eight healthy pregnant women were recruited for an evaluation of free T4 measured by different methods during each trimester of pregnancy, as compared with a group of 29 healthy controls.

Results

As with other studies, total T4 reached a plateau at 150 percent of non-pregnant values. Only the LC/MS/MS method showed the expected hCG-stimulated rise in free T4 in the first trimester (122% of non pregnant) as compared with immunoassay (95% of non pregnant). Relative to non pregnant, low free T4 immunoassay values were more common in the third trimester (81% of non pregnant) as compared with the LC/MS/MS method (92% of non pregnant). The correlation between the new LC/MS/MS method and the gold standard ED method across pregnancy was good (r = 0.89), but the correlation between the LC/MS/MS method and immunoassay was poor (r = 0.48).

Conclusion

The LC/MS/MS method is superior to immunoassay for generating trimester-specific free T4 reference values for pregnancy.

Commentary

It is becoming apparent that an adequate supply of maternal T4 is critical for optimal maternal health and fetal development. Unfortunately, non pregnant reference ranges for TSH and T4 do not apply to pregnancy, making assessments of thyroid status during pregnancy somewhat difficult. Specifically, thyroidal stimulation by hCG in early pregnancy results in higher T4 and lower TSH during the first trimester, whereas binding protein changes appear responsible for a lower free T4 as gestation progresses. However, the magnitude and prevalence of low free T4 values in the second and third trimesters appears method-related.

Most laboratories use automated free T4 immunoassay tests in preference to labor-intensive equilibrium dialysis (ED) reference methods. It is increasingly apparent that immunoassays for free T4 determinations are sensitive to binding proteins and merely 'estimate' the free T4 status. In fact, a recent study has shown that free T4 immunoassay values correlate better with total rather free hormone changes ( see Fritz et al. in Clin Chem 53:911, 2007 ).

This study used LC/MS/MS methodology ' the candidate free T4 international reference method. This methodology reported the expected physiological first trimester rise in free T4, whereas the immunoassay method did not. However, this method will not solve the problem of reliable free T4 measurements during pregnancy, because LC/MS/MS equipment is quite expensive (~$ 500,000) and the method necessitates first isolating free from protein-bound T4 by conventional techniques such as ED or ultra-filtration. Fortunately, as with non pregnant patients, TSH is the most sensitive indicator of thyroid status during pregnancy. When free T4 measurements are needed to manage pregnant hyperthyroid patients, a targeting of free T4 to the upper third of the non pregnant immunoassay reference range can compensate for the problem of low free T4 immunoassay values in pregnancy.

Summary and commentary prepared by Carole Spencer (Related to Chapter 14 of TDM)