thyroid hormones- defination,synthesis

Thyroid hormones

The human body’s thyroid hormones play a crucial role in regulating several physiological processes, including growth, development, metabolism, and general homeostasis. The thyroid gland, an essential endocrine organ situated in front of the neck, directly behind the larynx, is the main producer and secretor of these hormones. Thyroid hormones, which consist of triiodothyronine (T3) and thyroxine (T4), are essential for coordinating cellular processes and preserving the homeostasis of biological processes.

The thyroid gland’s complex and well-coordinated mechanism is the foundation of thyroid hormone production. This process begins when thyroid follicular cells absorb iodide ions (I-) from the circulation. This is an essential phase that depends on the thyroid follicular cells having enough dietary iodine. Thyroid follicular cells produce the big glycoprotein thyroglobulin and secrete it into the follicular lumen after iodide absorption. The next stages in the production of thyroid hormones use thyroglobulin as a substrate.

Monoiodotyrosine (MIT) and diiodotyrosine (DIT) molecules are created when the iodide ions are oxidized and integrated into the tyrosine residues in thyroglobulin. The production of T3 and T4 uses these iodinated tyrosine residues as precursor molecules. TPO, an enzyme, catalyzes coupling interactions between MIT and DIT molecules, which then result in the synthesis of T3 and T4 molecules in the follicular lumen.

Thyroglobulin is synthesized and then endocytosed back into thyroid follicular cells, where it is broken within lysosomes by proteases. T3 and T4 molecules are freed from thyroglobulin by this enzymatic cleavage, which permits them to enter the circulation. Thyroid hormones are released into circulation and are then transported to target tissues and organs throughout the body with the help of carrier proteins including albumin, transthyretin (TTR), and thyroxine-binding globulin (TBG).

Thyroid hormones have considerably more impacts than only helping with metabolism. The control of heart rate, cardiac output, and cardiovascular function is greatly influenced by these hormones. They also affect thermogenesis and the control of body temperature; changes in thyroid hormone levels are frequently linked to variations in basal metabolic rate and heat output. Thyroid hormones also have a significant impact on the growth and operation of the central nervous system, which affects mood regulation, mental health in general, and cognitive performance.

Thyroid hormones also have an impact on reproductive health as they regulate menstrual cycles, fertility, and levels of reproductive hormones. Thyroid hormones also affect bone density, skeletal growth, and muscular strength, which helps to maintain the integrity of the musculoskeletal system.

A complex feedback loop including the brain, pituitary gland, and thyroid gland controls the production and release of thyroid hormones. Thyrotropin-releasing hormone (TRH) is secreted by the hypothalamus in response to a decrease in circulating thyroid hormone levels. This triggers the pituitary gland to release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland, facilitating the production of thyroglobulin, the absorption of iodides, and the release of thyroid hormones. On the other hand, increased thyroid hormone levels help to maintain thyroid hormone homeostasis by preventing the release of TRH and TSH.

thyroid hormones

functions and effects of thyroid hormones:

  1. Metabolism Regulation: Thyroid hormones regulate metabolism by controlling the rate at which cells convert nutrients into energy. They increase the basal metabolic rate (BMR), which affects processes such as heart rate, body temperature, and energy expenditure.

  2. Development and Growth: Thyroid hormones are essential for normal growth and development, particularly in infants and children. They are crucial for proper brain development, bone growth, and maturation of the nervous system.

  3. Regulation of Body Temperature: Thyroid hormones help regulate body temperature by influencing metabolic processes that produce heat.

  4. Heart and Cardiovascular Function: Thyroid hormones have significant effects on heart rate and cardiac output. They help maintain normal heart function and regulate blood pressure.

  5. Digestive System Function: Thyroid hormones influence the function of the digestive system, including the absorption of nutrients from food.

  6. Muscle Function: Thyroid hormones are important for muscle function, including muscle strength and coordination.

  7. Reproductive Health: Thyroid hormones play a role in reproductive health, including menstrual cycle regulation and fertility.

Thyroid hormones synthesis:

There are many processes involved in the manufacturing of thyroid hormones within the thyroid gland. This is a summary of the procedure:

  1. Iodide Uptake: The first step in thyroid hormone synthesis is the uptake of iodide (I-) from the bloodstream by the thyroid follicular cells. Iodide is essential for the synthesis of thyroid hormones, as it is the key component of both thyroxine (T4) and triiodothyronine (T3).

  2. Thyroglobulin Production: Thyroid follicular cells synthesize and secrete a glycoprotein called thyroglobulin into the follicular lumen. Thyroglobulin serves as a scaffold for thyroid hormone synthesis.

  3. Iodination of Thyroglobulin: Within the follicular lumen, iodide is oxidized and attached to tyrosine residues on thyroglobulin molecules. This process, known as iodination or organification, forms monoiodotyrosine (MIT) and diiodotyrosine (DIT), which are precursors of T3 and T4.

  4. Coupling of Iodotyrosines: MIT and DIT molecules undergo coupling reactions catalyzed by thyroid peroxidase (TPO) enzyme within the follicular lumen. Coupling of one molecule of MIT with one molecule of DIT forms triiodothyronine (T3), while coupling of two molecules of DIT forms thyroxine (T4).

  5. Endocytosis and Proteolysis: Thyroglobulin containing T3 and T4 molecules is endocytosed back into thyroid follicular cells. Within lysosomes, thyroglobulin is proteolytically cleaved, releasing T3 and T4 into the cytoplasm.

  6. Release of Thyroid Hormones: T3 and T4 are released from thyroid follicular cells into the bloodstream, where they bind to carrier proteins such as thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin for transport to target tissues throughout the body.

  7. Regulation: The synthesis and release of thyroid hormones are tightly regulated by a negative feedback loop involving the hypothalamus, pituitary gland, and thyroid gland. When circulating levels of thyroid hormones are low, the hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to release thyroid-stimulating hormone (TSH). TSH then acts on the thyroid gland to increase iodide uptake, thyroglobulin synthesis, and thyroid hormone synthesis and release. When thyroid hormone levels are adequate, negative feedback inhibits the release of TRH and TSH, helping to maintain thyroid hormone homeostasis.


To sum up, thyroid hormones are master controllers of several physiological functions that have a significant impact on growth, development, metabolism, and general health. The thyroid hormones’ complex production, secretion, and control highlight how vital they are to maintaining the best possible health and function of the body. Thyroid hormones are essential modulators of human physiology, including everything from metabolism to cardiovascular health, mental processes to reproduction, and a vital component of both endocrine control and metabolic homeostasis.

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