Measuring free thyroxine levels in neonatal heel-prick samples.

Area of Science:

• Biochemistry
• Neonatal screening
• Endocrinology

Background:

• The Dutch neonatal screening for Congenital Hypothyroidism (CH) relies on thyroxine (T4) levels in blood spots.
• Thyroxine binding globulin (TBG) is measured to correct low T4 values, but the commercial kit is unavailable.
• Alternative methods, such as free thyroxine (fT4) assessment, are needed.

Purpose of the Study:

• To evaluate the Bio-Rad Neonatal Free Thyroxine (fT4) enzyme immunoassay (EIA) kit as a replacement for TBG measurements in CH screening.
• To assess the correlation of fT4 with T4 and the T4/TBG ratio in neonatal blood spots.
• To compare the diagnostic performance of fT4 and T4/TBG ratio using ROC curves.

Main Methods:

• Evaluation of the Bio-Rad Neonatal fT4 EIA kit.
• Correlation analysis between fT4, T4, and T4/TBG ratio in random and low T4 samples.
• Receiver Operating Characteristic (ROC) curve analysis comparing fT4 and T4/TBG ratio in CH patients and controls.

Main Results:

• fT4 measurements correlated significantly with T4 levels in random samples and proven CH patients.
• fT4 showed correlation with T4 and the T4/TBG ratio in blood spots with low T4.
• ROC curves indicated similar performance between the fT4 assay and the T4/TBG ratio.

Conclusions:

• The fT4 assay demonstrates comparable validity to the T4/TBG ratio approach for CH screening.
• Further studies with larger sample sizes are recommended to confirm these findings before altering the current CH screening protocol.