19 Thyroid Gland Flashcards
. • Describe how the activity of the thyroid gland is controlled. • Describe the effects of thyroid hormones on cells and the body as a whole. • Describe the consequences of over- and under- secretion of thyroid hormones.
Where is the thyroid gland located?
In front of larynx and trachea.
Below thyroid cartilage (Adam’s apple).
Isthmus crosses 2nd and 3rd rings of trachea (parathyroid glands sit behind)
Describe (in general terms the structure of the thyroid gland.
2 lobes, ISTHMUS joins lobes, bow tie shape
(parathyroid glands sit behind)
What is the first endocrine gland to develop embryologically?
Thyroid gland
How does the thyroid gland develop embryologically?
= EPITHELIAL PROLIFERATION at floor of pharynx at floor of tongue–> migrates to final position–> migrates down- passes in front of hyoid bone–> thyroglossal duct whichconnects tongue to gland- degenerates.
What is a follicular cell?
Cells-make up thyroid gland. =arranged in spheres (thyroid follicles)
What is colloid?
= deposit of thyroglobulin- fills follicles (=’extracellular)
How can you differentiate between parathyroid and thyroid gland cells histologically?
Parathyroid
=consists of ‘chief cells’-produce PTH
Thyroid
=Follicular cells produce Thyroid hormone
=Parafollicular cells produce Calcitonin
=Colloid stores Thyroglobulin
What are tetraiodothryonine (thyroxine) and triiodothyronine respectively?
T3 and T4 hormones
What are T3 and T4 formed from?
Two tyrosines + iodine
What’s the name of the protein that acts as a scaffold on which thyroid hormones are formed?
Thryoglobulin
What are the 3 reactions required for the generation of thyroid hormones from thyroglobulin proteins?
1) Oxidation- iodine-iodine
2) Addition of Iodine -to tyrosine acceptor residues on protein thyroglobulin
3) Coupling- tyrosines to generate thyroid hormones
Which enzyme regulates the 3 reactions required for the generation of thyroid hormones from thyroglobulin proteins?
Thyroid Peroxidase
Where can 90% of the iodine in the body be found?
Thyroid gland
In what form is iodine absorbed in the small intestine?
Iodide (reduced iodine)
How is iodide taken up from the blood by thyroid epithelial cells?
Sodium-iodide symporter (‘iodine trap’)
Give examples of common dietary sources of iodine.
Dairy products, grains, meat, vegetables
What is the relationship between T3 and T4?
- 90% secreted thyroid hormone= T4
- Biological activity of T3= 4x higher than T4
Where is most T4 converted to T3?
Liver and kidneys
How are T3 and T4 transported in the blood?
Bound to protein: THYROXINE BINDING GLOBULIN
In general terms, what effects do thyroid hormones have?
- Effect METABOLISM
- Effect GROWTH AND DEVELOPMENT
How does the negative feedback work in thyroid hormone regulation?
1- TSH negative feedback on hypothalamus
2- Thyroid hormone negative feedback on hypothalamus and anterior pituitary
What is the structure of TSH?
Glycoprotein hormone
How does TSH trigger Thyroid hormone release?
TSH stimulates: iodide uptake and oxidation, thyroglobulin synthesis, thyroglobulin iodination, colloid pinocytosis into cell, proteolysis- thyroglobulin, cell metabolism and growth
What are the 2 second messenger pathways induced by TSH?
Adenyl cyclase & Phopholipase C
What actions does thyroid hormone exhibit?
- INCREASE BASAL METABOLIC RATE AND HEAT PRODUCTION- increase number and size of mitochondria, stimulate synthesis of enzymes in respiratory chain
- STIMULATE METABOLIC PATHWAYS-lipid metabolism (lipolysis), carbohydrate metabolism- increase gluconeogenesis and glycogenolysis
- SYMPATHOMIMETIC EFFECTS- increase receptor number on CATECHOLAMINE target cells
What happens if the heart’s responsiveness to catecholamines is increased?
CO increases, increase peripheral vasodilation- carry heat to body surface
What effect does Thyroid hormone have on the nervous system?
Increase myelination of nerves and neutron development
How do Thyroid Hormone receptors function? (=in nucleus)
=Hormone-activated transcription factors- modulate gene expression. Bind DNA- in absence of hormone- transcriptional repression
What happens once Thyroid hormone has entered the cell?
Thyroid hormone= lipid soluble- enters cell through thyroid hormone transporters. Receptor=pre-bound to DNA sequences. Thyroid hormone binds- relieves repression- new protein expressed
Give an example of a thyroid hormone activated gene.
PEPCK, Sodium- potassium ATPase
Apart from TSH- what other variations are thyroid hormone levels sensitive to?
Albumin changes, dilution effects
