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16 Cards in this Set
- Front
- Back
How much does the thyroid weigh?
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15-20g (adult)
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Where is the thyroid located?
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Extends C5-T1
- oblique line on thyroid cartilage limites sup extent |
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What are the arteries of the thyroid?
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Superior thyroid
Inferior thyroid Thyroid ima (3%) Blood flow c 5x weight/min |
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What are the veins of the thyroid?
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Superior thyroid
Middle thyroid Inferior thyroid |
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What shape is the thyroid?
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Pyramidal lobe
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Describe the relations of the thyroid.
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- Forms part of median visceral column
- Invested by pretracheal fascia - Relationships include > recurrent laryngeal nerve > parathyroids |
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What is the surface anatomy of the thyroid?
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- Lateral lobes limited by oblique line of thyroid cartilage
- Isthmus 2nd and 3rd tracheal rings - palpate from behind |
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What are the methods used for thyroid imaging?
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Ultrasound
Nuclear medicine scan - Sodium pertechnetate NaTcO4 - Distributes as per iodine - Concentrates in thyroid - active transport - short half life - gamma emitter |
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Describe the foramen cecum.
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- Located in the anterior 2/3 posterior 1/3 tongue
- Diverticulum give rise to thyroid - Joined by ultimobranchial bodies (5th pouch) - form C (parafollicular) cells |
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Describe the histology of the thyroid.
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- Highly vascular
- Consists of aggregation of spherical follicles surrounded by dense capillary network - Colloid (glycoprotein 2x MW 335 000 ~ 70 tyrosine residues) enclosed within folliular epithelial cells - height of epithelium indicative of activity - size of follicles can vary widely most 200-300 (range 2-900 um) |
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What do C cells (follicular & interstitial) secrete?
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Calcitonin
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Describe the synthesis of thyroglobulin.
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Iodine trap
- Active transport sodium/potassium ATPase linked I- pump - Iodine concentration in thyroid about 30x concentration in blood - Very stimulated gland can be 250x - Iodine transported to apical membrane oxidised (nascent iodine I0) and iodinates (iodinase) tyrosine in thyroglobulin (called organification) - Synthesis thyroglobulin (660kd glycoprotein) about 140 tyrosine residues. - 20-30 of the Tyrosine residues in thyroglobulin iodinated mono & di-iodotyrosines - Iodination occurs at apical membrane of follicular cell |
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Describe thyroglobulin uptake and release of thyroid hormones.
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Endocytic (pinocytic) uptake of thyroglobin
- Digestion of thyroglobulin by lysosomal proteases - T4 and T3 released via base of cell - Mono and de-iodinated tyrosines deiodinated and I- recovered and recycled - Release ratio T4 (17) to T3 (1) - diffusion down concentration gradient |
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How is the thyroid regulated?
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TRH-tripeptide from hypothalamus
TSH-glycoprotein hormone about (28kd) - Proteolysis thyroglobulin (<30 miinutes) - stimulates iodide pump - tyrosine iodination up - hypertrophy of follicular cells - hyperplasia of follicular cells |
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How is thyroid hormone in the circulation?
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99% is bound
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Descrie the metabolism of thyroid hormone.
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Half lives
- T4 about 7 days - T3 about 1 day Inactivation by deiodination to inactive di and mono metabolites |