Thyroid Gland

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  • Created by: Becca
  • Created on: 24-12-13 18:37
What are the two hormones made in the thyroid gland?
T3 = tri-iodothyroninie & T4 = thyroxine
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How is thyroid hormone made?
Thyroid hormone is made by the synthesis and storage of thyroglobulin = a large gylcoprotein with many tyrosine molecules (+ iodination)
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How are T3 & T4 iodinated?
Tyrosine iodinated at 3' of ring = MIT (mono), 3' & 5' = DIT (di) -> DIT + DIT = thyroxine, T4. MIT + DIT = tri-iodothyronine, T3. DIT + MIT = REVERSE tri-iodothyronine, rT3 (biologically inactive, antagonist at T3 receptors)
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How are T3/4 synthesised?
Active uptake of iodide into follicular cell via Na/I symporter, movement across apical membrane, oxidation via thyroid peroxidase (TPO)->iodine. Iodination of tyrosine attached to thyroglobulin -> MIT+DIT. Coupling of MIT+DIT -> T3/4 (via TPO)
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How are T3/4 stored & released?
Storage of iodinate thyroglobulin (Tg) in colloid. Release: Tg taken into follicular cell by endocytosis, fusion with lysosomes -> proteolysis and release of T3/4 into plasma
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Why is there a problem when trying to store T3/4? What is the role of thyroid hormones?
They're too small! They increase metabolism in all cells (BMR), stimulate growth & development and are synergistic with actions of SNS/catecholamines (upregulate expression of B-adreoceptors)
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What is the regulation of T3/4?
TSH increases the size & number of thyroid follicular cells. Cold (neonate) increases TRH -> increase TSH -> increases T3/4 & metabolic rate. Acute stress directly inhibits TRH, increases somatostatin -> decrease TSH & T3/4 -> decreased metabolism
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How are thyroid hormones transported in circulation? How are circulating hormones used to measure thyroid function?
Lipophilic molecules transported bound to plasma protein molecules, amount of free hormone very small thus not much to bind to cells & influence hormonal levels (inert when bound). Measurement of free T4 indicates thyroid function (than total T4)
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What happens to T3/4 once it's in a target tissue? What is there role & potency?
T4 is converted to T3 or rT3 using deiodinase enzymes. T3 is the principally active hormone & is 5x more potent than T4, which is a prohormone
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Which factors can influence peripheral conversion of thyroid hormones?
Disease (kidney/liver), acute stress (surgery/illness/injury), some drugs, diet (low protein/starvation) etc -> decreased T3, increased rT3 -> lower metabolic rate
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What is the embryological developement of the thyroid gland?
Develops from an endodermal downgrowth of the floor of the pharynx between anterior 2/3 & posterior 1/3 of tonuge. Grows from foramen caecum (between 1st & 2nd pharyngeal arches) forming the thyroglosal duct
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What is the blood supply of the thyroid gland?
2(3) arteries: inferior thyroid & external carotid (thyroidea ima, directly from aorta/subclavian). 3 veins: superior/middle/inferior thyroid veins, drains -> internal jugular vein -> superior vena cava
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What is the histology of the thyroid cells & parafollicular cells? What is their role?
Thyroid cells: simple squamous epithelia surround colloid, acts as external storage. Parafollicular cells: large, eccentric nucleus, sit between thyroid follicles, produce calcitonin
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How does the appearance of epithelium change with the functional state??
Normal cuboidal cells have variable amounts of colloid. Inactive flatterned cells have large amounts of colloid. Hyperactive columnar cells have little lumen or colloid stored
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How many parathyroid glands are there? Where are they situated? What do they secrete?
4 parathyroid glands on the posterior surface of the thyroid gland, also migrate down neck. Secrete parathyroid hormone involved in calcium homeostasis, increases calcium in the blood (calcitonin decreases it)
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What is the secretory cell type in the parathyroid?
Chief cells. Other darker staining cells are of unknown function, called oxyphil cells
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Card 2

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How is thyroid hormone made?

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Thyroid hormone is made by the synthesis and storage of thyroglobulin = a large gylcoprotein with many tyrosine molecules (+ iodination)

Card 3

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How are T3 & T4 iodinated?

Back

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Card 4

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How are T3/4 synthesised?

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Card 5

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How are T3/4 stored & released?

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