Thyroid Hormones–Chemistry, Synthesis, Secretion, Functions and Regulations.

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 Thyroid hormones are essential for regulating various metabolic processes in the body, including energy production, growth, and development. The primary thyroid hormones are thyroxine (T4) and triiodothyronine (T3). Here's an overview of thyroid hormones, including their chemistry, synthesis, secretion, functions, and regulation:

1. Chemistry:

  • T4 (Thyroxine): T4 is the main thyroid hormone produced by the thyroid gland. It contains four iodine atoms and is less biologically active than T3.
  • T3 (Triiodothyronine): T3 is the more biologically active form of thyroid hormone. It contains three iodine atoms and is converted from T4 in peripheral tissues.

2. Synthesis:

  • Iodine Uptake: The thyroid gland actively takes up iodine from the bloodstream, a process regulated by the sodium-iodide symporter (NIS) on thyroid follicular cells.
  • Thyroglobulin Production: Thyroid follicular cells synthesize thyroglobulin, a large protein that serves as a storage form of thyroid hormone.
  • Iodination of Thyroglobulin: Iodine atoms are attached to tyrosine residues within thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).
  • Hormone Formation: Two DIT molecules combine to form T4 (thyroxine), while one MIT and one DIT combine to form T3 (triiodothyronine).
  • Release of Hormones: When stimulated by thyroid-stimulating hormone (TSH) from the anterior pituitary, thyroid follicular cells release T4 and T3 into the bloodstream by cleaving them from thyroglobulin.

3. Secretion:

  • Thyroid hormone secretion is regulated by a negative feedback loop involving the hypothalamus, anterior pituitary, and the thyroid gland.
  • When blood levels of T3 and T4 drop below normal (e.g., due to cold exposure, stress, or low dietary iodine), the hypothalamus releases thyrotropin-releasing hormone (TRH).
  • TRH stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH).
  • TSH acts on the thyroid gland, stimulating the production and release of T3 and T4 into the bloodstream.
  • Elevated blood levels of T3 and T4 exert negative feedback on the hypothalamus and anterior pituitary, reducing TRH and TSH secretion, respectively, and thereby regulating thyroid hormone levels.

4. Functions:

  • Thyroid hormones play a central role in regulating metabolism by influencing the function of virtually every cell in the body.
  • Key functions include:
    • Increasing basal metabolic rate (BMR) and heat production.
    • Enhancing the breakdown of nutrients (carbohydrates, fats, proteins) for energy production.
    • Promoting growth and development, particularly in children.
    • Regulating body temperature, heart rate, and overall metabolic homeostasis.

5. Regulation:

  • Thyroid hormone regulation involves the hypothalamus-pituitary-thyroid axis.
  • Negative feedback loops ensure that T3 and T4 levels remain within a narrow range.
  • Factors that can influence thyroid hormone levels include stress, cold exposure, illness, and certain medications.

In summary, thyroid hormones, primarily T4 and T3, play a central role in regulating metabolism and maintaining overall physiological balance in the body. Their synthesis, secretion, and functions are tightly regulated to ensure that energy production, growth, and development proceed optimally. Dysregulation of thyroid hormones can lead to various health issues, such as hypothyroidism or hyperthyroidism.

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