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Describe the arterial blood supply of the thyroid gland.
- - Superior Thyroid Artery (branch of external carotid)
- - Inferior Thyroid Artery (branch of thyrocervical trunk)
- - Ima (branch of aorta). Not seen in all patients.
Describe the venous drainage of the thyroid gland.
- - Superior Thyroid Veins (drains into internal jugular)
- - Middle Thyroid Veins (drains into internal jugular)
- - Inferior Thyroid Veins (drains into brachiocephalic vein)
Major nerves near thyroid gland?
- - Vagus nerves in carotid sheath
- - Recurrent laryngeal nerves medially. Left recurrent loops around aortic arch, right loops around right subclavian.
Stages of thyroid hormone synthesis and secretion?
- 1. Iodide Trapping & Conversion
- Iodide is actively transported into follicular cells and oxidised into iodine by H202 and thyroid peroxidase.
- 2. Thyroglobulin Synthesis
- Many tyrosine molecules form thyroglobulin (TGB) in ER.
- 3. Iodinisation of Tyrosine
- 1 iodine binding yields T1, 2 yields T2. Substance termed 'colloid'
- 4. Coupling
- T1 + T2 = T3
- T2 + T2 = T4
- 5. Endocytosis and Digestion
- Proteases in the lysozome break down TGB and cleave off T3 & T4.
- 6. Secretion
- T3 and T4 are lipid-soluble and diffuse into capillaries.
- 7. Iodine recycling
List the effects of T3 on the cardiovascular system:
- - Cardiac output
- - Tissue blood flow
- - HR
- - Myocardial contractility
- - Respiratory/ventilatory rate
List the effects of T3 on cellular metabolism:
- - Size and number of mitochondria
- - Na/K ATPase
- - Glucose absorption
- - Gluconeogensis
- - Lipolysis
- - O2 consumption
- - CO2 production
- - Protein synthesis
- - BMR
- - Ryanodine receptor expression
- - Ca2+ uptake from sarcoplasmic reticulum
Clinical tests for assessment of thyroid function?
- - Serum TSH
- - Free T3
- - Free T4
Describe the pathology of Graves' Disease
- IgG antibodies bind to TSH providing an agonistic effect; upregulating adenylyl cyclase and thus increasing thyroid hormone synthesis.
- Also other antibodies can be present but are cytotoxic, including TGB, TPO and the sodium-iodide transporter.
Describe the steps in thyroid hormone secretion from the hypothalamus, in addition to feedback mechanisms.
- 1. TRH released from hypothalamus.
- 2. TSH released from anterior pituitary.
- 3. TSH acts on follicular cells of thyroid gland to produce T3 and T4 which are secreted into blood and act on virtually all tissues in the body.
- 4. T3 levels sensed by both hypothalamus and ant.pituitary to provide negative feedback in order to regulate levels.
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