Which one of the following hormones contains iodine?

Thyroxine is the correct answer because it is a primary hormone produced by the thyroid gland, which requires iodine for its synthesis. The chemical structure of thyroxine consists of an amino acid core attached to four iodine atoms. In contrast, insulin is a peptide hormone made of amino acids, while testosterone and adrenaline are steroid and amine hormones respectively, none of which contain iodine. Iodine deficiency in the diet leads to a decrease in thyroxine production, often resulting in a condition known as goiter.