*3.1. Tablet Analysis*

Calibration data was generated for both T3 and T4 over the concentration range 0–250 ng/mL, based on 11 data points. Subsequently, the calibration curve obtained gave an r2 value of 0.9962 and 0.9973 for T3 and T4, respectively (Table 1). The developed LC–MS/MS method was both sensitive and selective for T3 and T4 analysis, with limit of quantitation (LOQ) values of 1.6 and 1.3 ng/mL and limit of detection (LOD) values of 0.2 and 0.8 ng/mL, respectively. Calculations were based on the standard curve method: LOD = (3.3σ)/S and LOQ = (10σ)/S, where σ is the standard deviation and S is the slope of the curve [20]. In addition, a quality control standard (50 ng/mL) was analyzed throughout the experimental duration and given average recoveries of 99.6% and 102.6% for T3 and T4, respectively. Tablet analysis was performed prior to dissolution testing to ensure that the stated dosage was present, as this would be used to indicate 100% of release under dissolution testing. The tablet analysis was consistent with the dosages stated on the packaging for all four batches of thyroid supplement with a % content of >93.3% for all batches and replicate preparations (Table 2). Good inter-batch and inter-tablet precision were also observed with % (relative standard deviation) RSD of <1.8% and <3.4%, respectively.

**Table 1.** Analytical data for T3 and T4 by LC–MS/MS.


LOD = limit of detection; LOQ = limit of quantitation.


**Table 2.** Tablet analysis: single- and dual-component thyroid hormone supplements.
