*2.3. Dissolution*

Dissolution was carried out using a SOTAX Smart AT7 ™ dissolution bath from Sotax (Finchley, London). The dissolution bath was set to 37 ◦C and 250 mL of fasted-state simulated gastric fluid (FaSSGF) was added to each vessel; the rotor speed set to 75 RPM and allowed to equilibrate for at least 1 h. The initial and final temperatures and pH were recorded to ensure consistent temperature and buffer control. Tablet weights were recorded prior to being released into the relevant vessels simultaneously. Samples were taken at the following time points through probe filters and syringe filters: 5, 10, 20 and 30 min. The paddles were stopped and 250 mL of fasted-state simulated intestinal fluid (FaSSIF), added to each vessel and the paddle was resumed. Samples were taken at the following time points through probe filters and syringe filters: 35, 40, 50, 60, 90, 120, 150, 180, 210 and 240 min.

#### *2.4. Preparation of Stock Solutions and Dissolution Media*

Stock solutions of each hormone were prepared at a concentration approximately 0.5 mg/mL in methanol, aliquoted into 100 μL aliquots and stored at −20 ◦C, as recommended by the manufacturer to increase the working life of the standard solutions. A fresh working standard solution of both standards was prepared each week by dilution of stock solutions in mobile phase (70% methanol: 30% water, *v*/*v*). Calibration standards were prepared daily for each analysis from the working stock solution ranging from 1 to 200 ng/mL for LC–MS/MS. A quality control standard containing both thyroid hormones was also prepared at a concentration of 50 ng/mL.

To prepare the fasted-state small intestinal fluid (FaSSIF) buffer, 0.84 g of sodium hydroxide pellet, 7.90 g of monobasic sodium phosphate monohydrate and 12.38 g of sodium chloride were added to approximately 1.8 L of Type 1 water. The solution was pH adjusted to 6.5 with 1 N sodium hydroxide and made up to 2 L with Type 1 water. To make the final FaSSIF medium, 4.48 g of FaSSIF/FeSSIF/FaSSGF powder was added to approximately 1 L of FaSSIF buffer. The solution was stirred until completely dissolved and made up to 2 L with FaSSIF buffer. The solution was allowed to stand for 2 h prior to use [19]. To prepare the fasted-state gastric fluid (FaSSGF) buffer, 3.2 g of sodium chloride was added to approximately 1.8 L of Type 1 water. The solution was pH adjusted to 1.6 with 1 N hydrochloric acid and made up to 2 L with Type 1 water. To make the final FaSSGF medium, 0.12 g of FaSSIF/FeSSIF/FaSSGF powder was added to approximately 1 L of FaSSGF buffer. The solution was stirred until completely dissolved and made up to 2 L with FaSSGF buffer [19].

### *2.5. Tablet Analysis*

For each batch of tablets, three tablets were weighed and crushed using a pestle and mortar. In triplicate for each tablet, a fifth of the weight was transferred to a 10 mL volumetric flask, made up to volume with water, and sonicated for 20 min. The solutions were allowed to cool, then 10 μL was

transferred to a separate 10 mL volumetric flask and made up to volume with water. The solution was then filtered through a 0.22 μm nylon filter and placed in autosampler vials for analysis by LC–MS/MS.

### **3. Results & Discussion**
