Semester 4, Case 9- anatomy
What encloses the thyroid gland?
Enclosed by a thin connective tissue capsule from which, septa project into the gland. A loose connective tissue sheath surrounds the gland and the capsule.
What is the name of the pit, on the dorsum of the tongue, which marks the site of origin of the thyroglossal duct?
Foramen caecum
Extra arterial supply to thyroid gland in 10% population
In 10% of the population a third artery also contributes. This vessel is termed the thyroid ima artery and it may arise from the brachiocephalic trunk or arch of aorta, less commonly is has been seen to arise from the right common carotid, subclavian or internal thoracic arteries.
How do you distinguish between inactive and active follicles?
In the inactive state the follicular cells are cuboidal in shape and have round nuclei. In active state, the cells appear taller and more columnar in shape and their nuclei appear paler.
What does thyroxine do?
Increases basal metabolic rate.
What artery are the parathyroid glands most commonly supplied by?
Inferior thyroid artery
Into which vein do the superior and middle thyroid veins empty?
Internal jugular vein
Structure of parafollicular cells
Nuclei are pale staining and appear slightly larger than the follicular cells. Their cytoplasm has a granular appearance.
What does the thyroid gland consist of?
Numerous rounded follicles containing colloid.
Ectopic thyroid glands
Occur when the gland fails to descend. In incomplete descent the gland will be located high in the neck. Rarely, the gland may fail to leave the tongue and a lingual thyroid develops.
What structures may be compressed by a thyroid goitre?
Oesophagus, recurrent laryngeal nerves and trachea.
What shows that the colloid has been taken back by the follicular cell?
Pale areas seen on the outer edge of the colloid lake.
What other cell type is present in the thyroid gland?
Parafollicular or C cells and they contribute to 2% of the cells within the gland.
Lateral lobe of the thyroid gland
The apex extends up to the oblique line of the thyroid cartilage and the base to the level of the fourth or fifth tracheal ring.
What do parafollicular cells secrete?
calcitonin
What do parathyroid glands secrete?
parathyroid hormone (PTH)
Which nerve crosses the inferior thyroid artery?
recurrent laryngeal nerve
Which vertebral level is the suprasternal notch located?
T2/3
Thyroid gland
An endocrine gland that is located in the anterior aspect of the neck.
Thyroid goitre
An enlargement of the thyroid gland which may occur due to over activity or under activity of the gland.
What is the thyroid gland made up of?
An isthmus and two lateral lobes. In some cases, a pyramidal lobe is also present.
Anterolateral relations of the thyroid gland
Anterior border of the sternocleidomastoid, sternohyoid muscle, superior belly of omohyoid, sternohyoid muscle.
Which directions does the thyroid gland spread when it is enlarged?
Anterior, posterior, inferior and lateral.
In how much of the population are pyramidal lobes in the thyroid gland present?
Approx 50%
Where do the superior parathyroid glands usually lie?
Below the point where the superior thyroid vessels enter the thyroid gland.
Into which vein does the inferior thyroid vein empty?
Brachiocephalic vein
What prevents the thyroid gland from enlarging in a superior direction?
By superior attachments of sternothyroid and sternohyoid muscles.
Which vertebral level is the hyoid bone located?
C3
Which vertebral level is the thyroid notch located?
C4
Which vertebral level is the thyroid gland located?
C5-T1
Which vertebral level is the lower border of cricoid cartilage located?
C6
Walls of each follicle
Consist of a single layer of cuboidal epithelial cells which secrete the colloid into the central colloid lake.
Medial relations of the thyroid gland
Cricothyroid muscle, recurrent laryngeal nerve, larynx, trachea, oesophagus, external laryngeal nerve, pharynx.
Isthmus of the thyroid gland
Crosses anterior to the 2nd, 3rd and 4th tracheal ring.
Development of pyramidal lobes
Develop from the remnants of the epithelium and connective tissue of the thyroglossal duct.
Development of the thyroid gland
Develops from a thickening on the floor of the embryonic pharynx in the developing tongue. A diverticulum develops from this thickening forming the thyroglossal duct. The thyroglossal duct elongates and its distal end becomes bi-lobed and proliferates into the thyroid gland. The thyroid gland descends through the neck anterior to the hyoid bone. At seven weeks the gland reaches its adult position. The thyroglossal duct subsequently degenerates but a pit remains on the dorsum of the tongue marking its site of origin.
Size and shape of parathyroid glands
Each gland is ovoid and approx 6mm long.
What happens to the colloid when it is in the colloid lake?
It is iodinated whilst it is within the colloid lake forming triiodothyronine (T3) and thyroxine (T4). It is later reabsorbed by the follicular cells and then secreted into the blood.
Where do the blood vessels supplying the thyroid gland lie?
Lie between the capsule and the fascial sheath.
Accessory thyroid tissue
May present anywhere along the route of descent of the thyroglossal duct. It occurs when isolated pockets of glandular tissue of the thyroglossal duct fails to degenerate.
Innervation of parathyroid glands
Postganglionic sympathetic fibres from the superior or middle cervical ganglia.
From which layer of cervical fascia is the sheath of the thyroid gland derived?
Pretracheal fascia
Pyramidal lobe of the thyroid gland
Projects upwards from the isthmus to the left of the midline.
Innervation of the thyroid gland
Receives sympathetic innervation. Post ganglionic sympathetic fibres arise in the superior, middle and inferior cervical ganglia and pass through the cardiac, superior and inferior thyroid plexuses to reach the gland. Vasomotor information is transmitted in these fibres.
What does calcitonin do?
Reduces plasma calcium levels.
Lymphatic drainage of the thyroid gland
Rich lymphatic network is found within the connective tissue septa of the thyroid gland. These vessels drain into the prelaryngeal, pretracheal and para tracheal nodes. Efferent vessels from these nodes go to the deep cervical nodes.
Where are parafollicular cells located?
Scattered in between the follicles and their surfaces do not touch the lumina of the follicle.
Posterior relations to isthmus of thyroid gland
Second, third and fourth rings of tracheal cartilages.
Anterior relations to isthmus of thyroid gland
Sternothyroid muscle, anterior jugular vein, sternohyoid muscle.
What does parathyroid hormone do?
Stimulates osteoclasts and increases plasma calcium levels.
From which vessel does the thyrocervical trunk arise from?
Subclavian
Which nerve runs alongside the superior thyroid artery?
Superior laryngeal nerve
Where might the inferior thyroid glands be located?
Superior mediastinum
Arterial supply of thyroid gland
Superior thyroid artery supplies the upper pole of each lobe whilst the inferior thyroid artery supplies the posteroinferior aspect. There is considerable anastomosis between vessels.
What do the follicle cells secrete?
The glycoprotein component of the thyroglobulin and it is iodinated within the colloid lake where it is stored until needed. When needed, the follicular cells remove some of the stored colloid from the lake, they detach the T3 and T4 and secrete it into the capillaries.
Venous drainage of thyroid gland
The superior, middle and inferior thyroid veins drain the superior, middle and inferior parts of the thyroid gland respectively. These vessels form a plexus on the anterior aspect of the gland.
How many parathyroid glands are there?
Usually 4, two superior and two inferior.
Posterolateral relations of the thyroid gland
Vagus nerve, common carotid artery, carotid sheath, internal jugular vein.
Where does the superior thyroid artery arise?
external carotid artery
What does the inferior thyroid artery arise from?
thyrocervical trunk