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Thyroid hormones are made by the thyroid gland and play an important role in regulating metabolism and growth. Iodine acts as a building block and the specific number of iodine atoms determines the type of the hormone: four for thyroxine (T4) and three for triiodothyronine (T3). The number of iodine atoms is critical for the functionality of thyroid hormones. Levothyroxine and liothyronine (synthetic forms of thyroid hormones T4 and T3) are essential components of thyroid tablets. T4 is less active and needs to convert to the more active T3 to be fully effective. Accurate iodine determination in thyroid tablets is a crucial quality control measure, ensuring the effectiveness and safety of thyroid treatments.

A robust method is introduced for indirect determination of total iodine content in thyroid tablets as iodate, according to United States Pharmacopeia (USP) guidelines using the 884 Professional VA and the Multi-Mode Electrode pro.

Commercially available thyroid tablet containing 100 μg levothyroxine and 20 μg liothyronine.

884 Professional VA.
Figure 1. 884 Professional VA.

Sample preparation and the determination of iodine is carried out according to the USP monograph «Thyroid Tablets». The process involves dry ashing of the tablets, where organically bound iodine is released and later converted to iodate. The iodate content is determined with the 884 Professional VA (Figure 1) by differential pulse polarography.

  • Working electrode: Multi-Mode Electrode pro
  • Reference electrode: Ag/AgCl/KCl (3 mol/L)
    reference electrode with electrolyte vessel.
    Bridge electrolyte: KCl (3 mol/L)
  • Auxiliary electrode: Platinum rod electrode
Table 1. Parameters for IO3 determination
Parameter Setting
Working electrode DME
Mode DP – Differential Pulse
Start potential -0.8 V
End potential -1.5 V
Potential step 0.005 V
Potential step time 1 s
Pulse amplitude 0.05 V
Peak potential Iodate -1.18 V
Determination of iodate in a thyroid tablet by differential pulse polarography with the 884 Professional VA and the Multi Mode Electrode pro.
Figure 2. Determination of iodate in a thyroid tablet by differential pulse polarography with the 884 Professional VA and the Multi Mode Electrode pro.

Calculation of the results was carried out according to the USP monograph «Thyroid Tablets».

Table 2. Results of iodine determination with the 884 Professional VA and the Multi-Mode Electrode pro.
Sample Iodine in μg / tablet Recovery rate
Tablet 70.59 92.3%
Tablet spiked with 72.55 μg 144.58 101.9%
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Internal reference: AW VA CH-0633-042024