Within this dissertation, synthetic, structural and stability studies in the series thyrostatic methimazole (1) were performed. By applying chemo- and regioselective chemical transformations, new, but also some known related substances of methimazole were prepared, ie. its known but also unknown potential impurities. In particular, the stability of methimazole in chlorinated solvents such as dichloromethane and dichloroethane, ubiquitous in chemistry and pharmaceuticals, has been studied. The obtained bridged bis methylene and ethylene derivatives were isolated and structurally characterized, and the characterization and transformation of their solid forms was performed. In this context, special emphasis is placed on the chemistry of 1,2-bis [(1-methyl-1H-imidazol-2-yl) thio] ethane (3a) formation which includes an attempt to detect and isolate a possible thiiranium (episulfonium) intermediate (8). Based on the conducted synthetic, spectroscopic, thermal, diffraction and computational studies, including kinetic measurements, intermediate 8 was not found but its isomer 7-methyl2H, 3H, 7H-imidazole [2,1-b] thiazol-4-ium chloride (7a) is to our knowledge the first known stable isomer of thiiranium ions. It was also found that 3a was formed directly by nucleophilic substitution of the primarily formed intermediate chlorine ethyl derivative, 2-[(chloroethyl) thio]-1-methyl-1H-imidazole, 6) with methimazole. Oxidation and methylation derivatives of 1 have also been prepared and characterized, and in the context of known methimazole impurities B and C, samples have been prepared and standardized. As part of the preformulation studies, a solid state study of 1 was performed in the presence of selected excipients. High stability of 1 in the planned pharmaceutical formulations was determined, but also a new unknown, dimer and trimer of methimazole were found and characterized, as its potential impurities. At the same time, the chemistry of their formation was proposed. The results of the study complements the current knowledge about methimazole (1) which provides a comprehensive insight into the properties of its structural analogues, as potential impurities. This contributes to increasing the quality, safety and efficacy of both methimazole and its final dosage forms.