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).


Know the definition of Hypothyroidism and the clinical significance of Goiter (see end of Lecture notes).

The Thyroid Hormones:

Iodine: All you need to know about I...................

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.

Iodine Metabolism and Balance in Humans: SKIP this picture.

Synthesis of Thyroid Hormones: Know the 7 steps in the synthesis of T3 and 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

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.

Principal Function- "spark" for the oxidative reactions and regulation of metabolic rates in the body

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