# A Rapid Fluorescence Quenching Assay for Total Levothyroxine Quantification in Pharmaceutical and Supratherapeutic Serum Samples

## TL;DR

A new quick and affordable method uses fluorescence to measure levothyroxine in drugs and blood samples, offering a simpler alternative to existing techniques.

## Contribution

A novel fluorescence quenching method for levothyroxine quantification with high accuracy and minimal interference.

## Key findings

- The method has linear detection ranges of 8.00 × 10⁻⁶ – 6.40 × 10⁻⁵ mol/L and low detection limits.
- Recovery rates in spiked human serum were 102–104%, indicating high accuracy.
- Common drugs and ions showed minimal interference, making the method reliable for real-world samples.

## Abstract

A rapid, cost-effective spectrofluorometric method was developed for levothyroxine (T4) detection using fluorescein isothiocyanate (FITC) as a fluorescent probe. The method exploits FITC fluorescence quenching by T4 in Tris/phosphate buffers (pH 7.4),with linear ranges of 8.00 × 10⁻⁶ – 6.40 × 10⁻⁵ mol/L and detection limits of 9.60 × 10⁻⁶ mol/L (Tris) and 8.80 × 10⁻⁶ mol/L (phosphate). Stern-Volmer analysis confirmed dynamic quenching dominated by electrostatic interactions. The method demonstrated 102–104% recovery in spiked human serum and minimal interference from common drugs (Ciprofloxacin, Alendronate) and ions (Fe³⁺, Al³⁺). Compared to HPLC and kinetic methods, this approach offers simplicity, speed, and accessibility for routine pharmaceutical/clinical analysis.

The online version contains supplementary material available at 10.1007/s10895-025-04510-9.