The Thyroid Gland: Its Evolution, Functions, and Impact on Growth and Metabolism

In 1656, the English anatomist Thomas Wharton first identified the thyroid gland. Later, in 1909, Theodor Kocher, a Swiss surgeon, received the Nobel Prize for his work in describing the gland's functions. Kocher successfully removed enlarged thyroid glands from patients between 1874 and the following decade, significantly reducing the mortality rate of the operation. However, patients reported fatigue, lethargy, and excessive coldness following the procedure. We now understand that the thyroid gland plays a critical role in various life cycle events, including energy utilization, growth and development, metamorphosis, reproduction, hibernation, and heat generation. It is among the largest of the endocrine glands and is present in all vertebrates, located in the neck region of tetrapods.

Thyroid hormones increase the metabolic activity of nearly all body tissues and accelerate the utilization of food as an energy source. The release of hormones increases the number and size of mitochondria, intracellular bodies that generate adenosine triphosphate (ATP) to provide energy for cellular functions and generate body heat. The effect of thyroid hormones on growth is primarily observed in children, as those with hypothyroidism have delayed and stunted growth and mental retardation.

The role of thyroid hormones is crucial in the growth and development of vertebrates, particularly in the transformation of the tadpole into an adult frog, which is a remarkable example of this phenomenon.

The thyroid gland's functions have evolved differently in various species over time. In 1912, J. Frederick Gudernatsch discovered that feeding mammalian thyroid to tadpoles at Cornell University's medical school induced metamorphosis. This process transformed tadpoles into adult frogs, with external gills disappearing, a large jaw developing, the eyes and legs growing rapidly, and the tail being resorbed. Removal of the thyroid gland from tadpoles, however, prevented metamorphosis. Flatfish also undergo metamorphosis, with one eye moving to the other side and becoming the upper side of the fish.