#13357 - How Is Thyroid Gland Structure Organised In Relation To Its Function - Organisation of the Body

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 the large amount of trafficking occurring. The activity of these follicular cells can be directly related to the appearance of its structure under the electron microscopes. Active epithelial cells secreting large amounts of thyroid hormones appear to be tall and the colloid is reduced in size. Whereas inactive glands the cells are low cuboidal and the follicles are filled with large amounts of colloid.

Symptoms if a patient was deficient in iodine.

They body is unable to synthesise iodine so it is an essential part of the diet. Common sources of dietary iodine include cheese, iodized table salt, salt water fish and eggs. Therefore iodine deficiency is mostly due to malnutrition and as the main role of iodine is in the production of the thyroid hormone symptoms often include an enlargement of the thyroid, hypothyroidism and retardation in infants if mothers were deficient during pregnancy. However to understand the consequences of a lack of iodine in the body it is important to consider what the effects of thyroid hormones are.

The two hormones secreted by the thyroid gland are tri-iodothryonine (T3) and thyroxine which is also known as tetra-iodothryonin. Both these hormones are based on 2 tyrosine amino acids that are covalently attached to iodine. A large proportion of the thyroid secretion consists of thyroxine which has four iodine atoms attached but the metabolically active thyroid hormone is tri-iodothryonine which has three iodine atoms attached. Therefore the inactive T4 is converted into T3 either in the blood plasma or in the cell via deiodinase enzymes.

T3 hormone acts universally in the body where its main effect is increasing the basal metabolic rate, hence increasing oxygen consumption and heat production. Once the T3 has entered the cells through the monocarboxylate transporter 8 it binds to receptors in the nucleus which results in the upregulation of transcription of specific genes. This results in an increased production of Na/K+ ATPase which uses 20-40% of ATP production within a cell. The T3 hormone also results in an increase in the metabolism of proteins, carbohydrates and lipids. In the case of cardiovascular system T3, increases the expression of myosin, B1 adrenergic receptors and Ca²⁺ ATPase. These changes lead to...