How Is Thyroid Gland Structure Organised In Relation To Its Function Notes

How is thyroid gland structure organised in relation to its function? Explain the likely symptoms if a patient were deficient in iodine.

Structure of the thyroid gland relating to its function

The thyroid gland is found anterior to the 2^nd to 4^th collagneous costal rings of the trachea and is loosely attached to it by a pretrachial fascia which gives of fine collagenous branches that divides the gland into lobules. By having this loose attachment it enables the thyroid to move on swallowing which makes it easy to notice if there is an abnormal enlargement of the gland. The thyroid gland consists of two lateral lobes which are found on either side of the larynx and trachea. The thyroid gland has a rich blood supply which is essential as it enables efficient transport of the hormones stimulating the gland and efficient removal of the hormones secreted by the gland. Arterial supply from the gland comes from the superior thyroid and inferior thyroid arteries which are branches from the common carotid whilst venous blood drains into the jugular veins.

The main function of the thyroid gland is to store, synthesise and secrete hormones. In order to maximise its specific role it has a unique structure. The functional units of the thyroid gland are the thyroid follicles which are spherical structures that have an outer layer composed of a single layer of cuboidal epithelium which is bound by a basement membrane. When these follicles are stained with haemotoxylin and eosin and viewed under an electron microscopy it is seen that the follicle lumen has a dense pink colour. Due to its pink staining, the material found in the lumen is known as colloid and biochemical investigations show that it contains iodinated thryoglobulin which is an inactive precurosor of the thyroid hormone. Having a large extracellular storage is functionally advantageous as it protects against nutrient deficiency and ensures that thyroid hormones can always be made even under conditions of starvation.

Having cuboidal epithelium surrounding the colloid is important as its main function is to synthesise iodinated thryoglobulin and replenish the stores. The follicular cells make hormones T3 and T4 in response to the TSH hormone released from the anterior pituitary gland. The TSH binds to GPCR on the basolateral surface of follicular cells which results in an increase in intracellular concentration of cyclic AMP. The rise in cyclic AMP stimulates the Na/K ATPase pump and this maintains a high extracellular sodium concentration which stimulates the sodium iodide symporter in the basal plasma membrane. This protein actively transports iodide from the blood into the follicular cells where it is oxidised to iodine via thryoperoxidase enzyme and is transported across the apical membrane and is secreted into the follicular lumen via the transporter pendrin. By having different proteins inserted into the basolateral and apical surfaces it enables these follicular cells to uptake and concentrate iodide from the blood and transport it into the lumen in a unidirectional manner.

Another important function of the cuboidal follicular cells is to synthesise the glycoprotein thryoglobulin. Thryoglobulin is first synthesised in the rough endoplasmic reticulum and is then transported to the golgi apparatus where it is packaged into vesicles and released across the apical surface into the follicular lumen by excocytosis. Once the iodine and thryoglobulin have entered the lumen of the follicle, the iodine binds to the tyrosine residues of the thryoglobulin to form inactive precursors of hormones T3 and T4.

When active forms of these hormones are required the follicular epithelial cells remove a segment of the colloid via pinocytosis and these cytoplasmic vacuoles then fuse with the lysomes. The hydrolytic enzymes of the lysosomes cleave the hormone from its inactive precursor. The active form of the hormone leaves the lysosome and is secreted out of the basal membrane into the capillaries.

An electron micrograph image of follicular cells shows how these cells are adapted to its synthetic function. These cells have large amounts of rough endoplasmic reticulum which enables large amount of glycoprotein thyroglobulin to be synthesised. They also have large dark prominent lysosomes which is where the active hormone is cleaved. Another feature is that these cells have nuclei that consist of dilated chromatin which indicates that active synthesis is occurring. Also seen in these images are a large number of synthetic vesicles which indicates...

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