Which of the following substances is NOT associated with the thyroid gland?

The substance that is not associated with the thyroid gland is somatotropin. Somatotropin, also known as growth hormone, is produced by the anterior pituitary gland and plays a crucial role in growth, metabolism, and muscle development. Its primary functions are related to growth and regulation of body composition rather than thyroid function.

In contrast, the other substances listed are directly associated with the thyroid gland. Thyroxine and triiodothyronine are hormones synthesized and secreted by the thyroid gland, responsible for regulating metabolism, energy production, and overall growth and development. Thyrotropin, also known as thyroid-stimulating hormone (TSH), is produced by the pituitary gland and stimulates the thyroid gland to produce thyroid hormones, linking it directly to thyroid function.

This distinction highlights how somatotropin operates outside the specific hormonal function of the thyroid gland.