2. The Thyroid Gland Flashcards Preview

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Flashcards in 2. The Thyroid Gland Deck (24):

Where is the thyroid gland?

-lies against and around the front larynx and trachea
-below the thyroid cartilage (Adams apple)
-isthmus extends from the 2nd to 3rd rings of the trachea


How is the thyroid derived embryologically?

- first endocrine organ to develop
- at 3-4 weeks gestation the thyroid appears as an epithelial proliferation in floor of the pharynx at base of tongue and takes several weeks to migrate to final position
-1st descends as diverticulum through the thyroglossal duct
-during migration remains connected to the tongue by thyroglossal duct which subsequently degenerates
-detracted thyroid then continues to its final position over following 2 weeks


Describe the histology of the thyroid tissue

-thyroid tissue is composed of follicular cells arranged in spheres known as thyroid follicles
-follicles are filled with colloid - deposit of thyroglobulin
-colloid is extracellular even though it is inside the follicle


What is secreted by the cells of the thyroid?

-thyroid follicular cells produce thyroid hormone
-thyroid parafollicular cells produce calcitonin


In the parathyroid gland, which cells produce parathyroid hormone?

The parathyroid principal also called chief cells


How is thyroid hormone synthesised?

-thyroid hormone consists of tyrosine residues which are iodinated (iodine group added to them)
- 1 iodine : MIT :monoiodotyrosine
-2 iodines: DIT : diiodotyrosine

MIT + DIT = triiodothyronine (T3)
DIT + DIT = tetraiodothyronine (T4) AKA thyroxine


What is thyroglobulin?

Thyroglobulin acts as a scaffold on which thyroid hormones are formed.

Thyroglobulin contains 134 tyrosine, although only a handful of these are actually used to syntehsise T3 and T4


What is thyroid peroxidase?

Membrane bound enzyme which regulates 3 separate reactions involving iodide.

1. Oxidation - iodide become iodine
2. Addition of iodine - to tyrosine acceptor residues o the protein thyroglobulin
3. Coupling - of MIT or DIT to generate thyroid hormones with the thyroglobulin protein


What happens to dietary iodine?

-dietary iodine is reduced to iodide before absorption principally in the small intestine before absorption
-thyroid hormones and precursors are the only molecules in the human body that contain iodine
-thyroid gland 95% iodine


How is iodide taken up?

-iodide taken up from the blood by thyroid epithelial cells which have a sodium iodide symporter or 'iodine trap'


What are the common sources of iodine?

- dairy products, grains, meat, eggs


What is the major form of thyroid hormone which is secreted?

-90% of thyroid hormone secreted is T$
-biological activity of T3 is 4 times that of T4
-most T4 will be converted to T3 in the liver and the kidneys
- 80% of circulating T3 is from T4
- T3 and T4 are transported in the blood bound to the protein thyroxine - binding globulin


How is thyroid hormone secretion regulated?

- via negative feedback
-thyroid hormone has effects on metabolism and also on growth and development
- Hypothalamus releases Thyrotropin releasing hormone (TRH)
-Anterior pituitary releases Thyroid stimulating hormone (TSH)
- thyroid gland will release thyroid hormone (TH)


What is the structure of TSH?

-glycoproteins composed of 2 non covalenty bound subunits (alpha and beta)
-the alpha subunit is also present in FSH and LH
-The beta subunit provides unique biological activity


What does TSH do to trigger thyroid hormone release?

TSH stimulates:
-iodide uptake
-iodide oxidation
-thyroglobulin synthesis
-thyroglobulin iodisation
-colloid pinocytosis
-prteolysis of thyroglobulin


Which pathways is TSH coupled?

The TSH receptor can be coupled to GaS and also Gaq.


What are the general effects of the thyroid hormone?

- increase in BMR and heat production
-stimulation of metabolic pathways - mainly catabolic
-sympathomimetic effects


What are the effects of thyroid hormone on the CVS system?

- increase hearts responsiveness to catecholamines
-increased cardiac output, increase in heart rate and increase in force of contraction
-increase in peripheral vasodilation to carry extra heat to body surface


What is the role of thyroid hormone in the nervous system?

-essential for both development and adult function
-myelination of nerve and development of neurones


What are the thyroid hormone receptors?

-nuclear receptors
-hormone activated transcription factors
-modulate gene expression
-when there is no hormone, the TH receptor binds to DNA leading to transcriptional repression
-when hormone binds, conformational change - transcriptional activator


What happens when thyroid hormone binds to an intracellular receptor?

-lipid soluble and enters cell through thyroid hormone transporter
-TH receptor is pre bound to the hormone response element on DNA
-when TH enters nucleus and binds to TH receptor it relieves repression and gene expression
-examples of TH activated genes included PEPCKL
-expression of new protein mediates the effects of thyroid hormone


Normal plasma levels of thyroid hormones

Free T4 : 0.7-2.1
Free T3 : 0.2 -0.5
TSH - 0.3 - 4.0

Free means that the hormone is not bound to plasma protein


What is thyroid scintigraphy?

-technetium 99m used for isotope scanning of the thyroid with a gamma camera
-most commonly used medical radioisotope
-biological half life 1 day
-low radiation exposure
-also used for brain scans, myocardial perfusion imaging and brain imaging


What is carbimazole?

-pro drug and is converted to methimazole in the body
-prevents thyroid peroxidase from coupling and iodisation tyrosine on thyroglobulin
-block the formation of thyroid hormone
-used to treat overactive thyroid (hyperthyroidism) e.g. Graves' disease