The Thyroid Hormone System- Study Guide

Know the definition of Hyperthyroidism and the clinical significance of Graves Disease and Goiter (see end of Lecture notes).

1. The crucial element and the key to the thyroid hormones.

2. Since there is little iodine in the body or from the diet, the thyroid gland can both concentrate iodine and make thyroid hormone.

3. Iodine from the diet is generally converted to iodide and absorbed by the gut.

4. Note: 90% of the body iodine is present in the thyroid, primarily as T₄.

i) iodide enters the thyroid by a pump linked to the Na⁺ / K⁺-ATPase

ii) hydrogen peroxide converts iodide into ("active") iodide I⁺

iii) iodination of Tyr produces MIT and DIT

iv) MIT and DIT are stored in the thyroglobulin

v) when thyroglobulin is cleaved by a protease, the MIT and DIT are deiodinated and the iodine recycled

vi) T₄ and T₃ are released to the blood

vii) Tyr and thyroglobulin are recycled

Regulation of TH Release: SKIP the picture but understand T₄ Feedback Regulation

1) inhibits TRH release from the hypothalamus

2) inhibits TSH release from the pituitary by limiting the sensitivity of the pituitary to TRH stimulation

Biological Effects and Mechanism of Action of Thyroid Hormones: The Thyroid hormone work through a Nuclear Receptor To: Increase Growth (via GH), Increase Basal Metabolic Rate (via GA-3-P DH) and Oxygen Consumption and to Regulate Body Temperature (via Na-K-ATPase). SKIP the picture.

T₃ is ten times more active than T₄

T₃ also acts two to three times faster than T₄

T₃ is the tissue active form of the thyroid hormone

© Dr. Noel Sturm 2013