What Are the Functions of Thyroxine?

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  • Written By: Rebecca Harkin
  • Edited By: Allegra J. Lingo
  • Last Modified Date: 08 December 2018
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The functions of thyroxine in the body are incredibly wide ranging. Thyroxine, also known as T4, plays at least some role in controlling basal metabolic rate (BMR), energy production, the cardiovascular system, bone heath, the central nervous system, the reproductive system, growth and development, and the digestive system. It is an essential hormone in the body, needing replacing or controlling when the thyroid fails to maintain a proper concentration in the body.

Basal metabolic rate is the amount of energy required by the body in a day. Thyroxine controls BMR, which it does by controlling the speed of metabolism and the amount of energy released. Inside each cell are tiny, energy-producing organelles called mitochondria. This hormone controls BMR by increasing the concentration of mitochondria in a cell and by increasing the energy produced by mitochondria.

Another function of thyroxine is to produce energy in the body, and this is done by controlling lipid, carbohydrate, and protein metabolism. T4 accelerates fat mobilization, or the conversion of fat cells into fatty acids and glycerol, which can be used as energy in the body. Carbohydrate metabolism is also enhanced by this hormone. The metabolism of carbohydrates results in a supply of powerful, quick-acting energy for the body. Thyroxine also controls both protein synthesis and the breakdown of protein.


Thyroxine plays many roles in the cardiovascular system. This hormone enhances the contraction of the heart, the heart rate, and the output of the heart. Vasodilation, or the opening of blood vessels, is also augmented by it.

This hormone also helps to control bone turnover by acting on both the osteoclasts, bone cells that remove bone, and osteoblasts, bone cells that replace bone. This breakdown and rebuilding cycle is essential to healthy bone. Thyroxine also helps control the accumulation and removal of calcium as well as the size of the bone calcium compartment or storage area of calcium within the bone.

One of the functions of thyroxine in the central nervous system is to control mental alertness. To much of the hormone will produce excitability and anxiety, while insufficient amounts will produce lethargy. Mental control may be due to the influence of throxine on catecholamine, one of the fight or flight hormones.

In the reproductive system, thyroxine helps to keep the menstrual cycle regular and plays a role in conception and the release of breast milk. It works in the digestive system to help increase the secretion of digestive enzymes in the stomach. Here, it also enhances the contraction of involuntary stomach muscles, promoting better digestion.

The physiological process of mental and physical growth in children is complex. The role of thyroxine in this process is not completely understood, but it clearly plays a role. Children with T4 deficiencies will often suffer from stunted growth and mental retardation. It also enhances hydration of the skin as well as hair growth.


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