Thyroid Hormones (TSH, T3, T4)

Thyroid Hormones (TSH, T3, T4)

Definition: Thyroid hormones (TSH, T3, T4) are hormones produced by the thyroid gland, primarily thyroxine (T4) and triiodothyronine (T3). These hormones play critical roles in regulating metabolism, growth, and development.

Types of Thyroid Hormones

• Thyroxine (T4):

• Structure: Contains four iodine atoms.
• Function: T4 is the primary hormone secreted by the thyroid gland. It is converted into the more active form, T3, in various tissues.

• Triiodothyronine (T3):

• Structure: Contains three iodine atoms.
• Function: T3 is the more active form of thyroid hormone and is responsible for most of the biological effects attributed to thyroid hormones.

Functions of Thyroid Hormones

• Metabolism Regulation:

• Thyroid hormones increase the basal metabolic rate (BMR), influencing how the body uses energy.
• They enhance glucose metabolism and lipid metabolism, promoting fat oxidation.

• Growth and Development:

• Essential for normal growth and development in children, particularly for brain development.
• Influence bone growth and maturation.

• Cardiovascular Effects:

• Increase heart rate and cardiac output by enhancing the sensitivity of the heart to catecholamines.

• Thermoregulation:

• Help regulate body temperature by increasing heat production through metabolic processes.

Regulation of Thyroid Hormone Levels

• Hypothalamic-Pituitary-Thyroid Axis: The secretion of thyroid hormones is regulated by thyroid-stimulating hormone (TSH), which is produced by the pituitary gland in response to thyrotropin-releasing hormone (TRH) from the hypothalamus.
• Feedback Mechanism: High levels of T3 and T4 inhibit TSH and TRH secretion, maintaining hormonal balance.

Clinical Relevance

• Hypothyroidism: A condition characterized by insufficient production of thyroid hormones, leading to symptoms such as fatigue, weight gain, and depression.
• Hyperthyroidism: Excessive production of thyroid hormones can cause symptoms like weight loss, increased heart rate, and anxiety.
• Thyroid Disorders: Conditions such as Hashimoto’s thyroiditis (an autoimmune disorder) and Graves’ disease can significantly impact thyroid hormone levels.

Conclusion

Thyroid hormones are vital for regulating metabolism, growth, and overall health. Understanding their functions and regulation is essential for diagnosing and managing thyroid-related disorders.

References

1. Braverman LE, Cooper DS. “Graves’ Disease.” New England Journal of Medicine. 2016; 375(15): 1456-1467. DOI: 10.1056/NEJMra1510020
2. Jameson JL, et al. “Disorders of the Thyroid Gland.” In: Harrison’s Principles of Internal Medicine. 20th ed. McGraw-Hill; 2018.
3. Pearce SHS, et al. “Diagnosis and Management of Thyroid Disease.” BMJ. 2016; 355: i5580. DOI: 10.1136/bmj.i5580