Thyroid Hormone Feedback Mechanism

Overview

• The lecture discusses the feedback mechanism of thyroid hormone release.
• Focus on the hypothalamus, pituitary gland, thyroid gland, and diagnostic implications.

Mechanism of Thyroid Hormone Release

1. Hypothalamus Role

   • Releases TRH (Thyrotropin-releasing hormone).

2. Pituitary Gland Role

   • Thyrotrope cells in the pituitary receive TRH.
   • Release TSH (Thyroid-stimulating hormone) into the bloodstream.

3. Thyroid Gland Role

   • TSH induces the thyroid gland to release thyroxine (T4).
   • Also releases T3, but T4 is the focus here.

4. Feedback Loop

   • T4 enters the bloodstream.
   • Feeds back to:
     • Pituitary: Reduces TSH release.
     • Hypothalamus: Reduces TRH release.

Clinical Implications

• Blood levels of thyroxine and TSH can diagnose thyroid problems.

• Diagnosis Table:

  Thyroxine (T4)	TSH	Diagnosis
  High	High	Pituitary issue (Feedback failure)
  Low	High	Thyroid issue (Non-responsive to TSH)
  High	Low	Thyroid issue (Releasing high T4 despite low TSH)
  Low	Low	Pituitary issue (Non-responsive to low T4 feedback)

Key Terms

• Hyperthyroidism: High thyroxine levels.
• Hypothyroidism: Low thyroxine levels.

Conclusion

• Understanding the feedback mechanism is critical for diagnosing the origin of thyroid issues based on hormone levels.