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16 Cards in this Set

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How much does the thyroid weigh?
15-20g (adult)
Where is the thyroid located?
Extends C5-T1
- oblique line on thyroid cartilage limites sup extent
What are the arteries of the thyroid?
Superior thyroid
Inferior thyroid
Thyroid ima (3%)
Blood flow c 5x weight/min
What are the veins of the thyroid?
Superior thyroid
Middle thyroid
Inferior thyroid
What shape is the thyroid?
Pyramidal lobe
Describe the relations of the thyroid.
- Forms part of median visceral column
- Invested by pretracheal fascia
- Relationships include
> recurrent laryngeal nerve
> parathyroids
What is the surface anatomy of the thyroid?
- Lateral lobes limited by oblique line of thyroid cartilage
- Isthmus 2nd and 3rd tracheal rings
- palpate from behind
What are the methods used for thyroid imaging?
Ultrasound
Nuclear medicine scan - Sodium pertechnetate NaTcO4
- Distributes as per iodine
- Concentrates in thyroid - active transport
- short half life
- gamma emitter
Describe the foramen cecum.
- 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
Describe the histology of the thyroid.
- 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)
What do C cells (follicular & interstitial) secrete?
Calcitonin
Describe the synthesis of thyroglobulin.
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
Describe thyroglobulin uptake and release of thyroid hormones.
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
How is the thyroid regulated?
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
How is thyroid hormone in the circulation?
99% is bound
Descrie the metabolism of thyroid hormone.
Half lives
- T4 about 7 days
- T3 about 1 day

Inactivation by deiodination to inactive di and mono metabolites