- Anatomy of thyroid gland
- Embryological development
- Physiology of thyroid hormone secretion
ANATOMY
The thyroid gland weighs about 30-40 gram in the adult and is slightly heavier in the female. It measures 50-60 mm in breadth and its anteroposterior dimension is 18-20 mm. It is relatively larger in the fetus and the infant. It is the largest endocrine gland in the body and is situated in the neck, in front of the trachea and on the lateral walls of the larynx. Part of the gland is in contact with the thyroid cartilage and hence its name.
The weight of the gland, its strategic positioning in the neck, its high vascularity and the storage of preformed thyroid hormones reflects the importance of the thyroid gland and its secretion triiodothyronine (T3) and thyroxine (T4). Receptors for these hormones are present in virtually all organs, underlining the uniqueness of the gland.
It consists of the right and left lobes and the isthmus that lies transversely joining the lower ends of the two lobes. The pyramidal lobe, a thin process, extends upwards from the isthmus and may reach the hyoid bone. The upper parts of the lateral lobes overlap the external surface of the thyroid cartilage and cover its inferior horn and the adjacent area. Inferiorly, the lobes extend to the 5th or 6th tracheal ring. The posterior surface of the isthmus is in contact with the second and third tracheal rings and the superior margins sometimes extend to the cricoid cartilage. Posteriorly, the lateral lobes are intimately related to the walls of the pharynx and esophagus and thus, a thyroid swelling can lead to dysphagia due to esophageal compression. Laterally, the lobes slightly extend over the anterior surface of the common carotid arteries. The outer surface of the thyroid is convex, the inner surface towards the trachea is concave.
In front, the thyroid is covered with skin, subcutaneous fat and the fascia of the neck (which provides the gland with 3an external capsule) and muscles (sternohyoid, sternothyroid and omohyoid). The thickening of the external capsule forms the ligament that joins the gland to the trachea and larynx, as a result of which it moves together with these organs. The capsule gives off processes into the tissue of the gland that separates it into lobules consisting of vesicles containing colloid, its chief component being thyroglobulin.
The thyroid gland receives two superior thyroid arteries (branches of external carotid arteries), two inferior thyroid arteries (branches of thyrocervical trunk, subclavian artery or even arch of aorta). The veins form a plexus, which lie under the external capsule and drain into three veins on either side-superior, middle and inferior thyroid veins. The superior and middle veins drain into the internal jugular vein and the inferior, into the brachiocephalic vein (Fig. 1.1).
Fig. 1.1: Anatomical relations of the thyroid gland(from Bailey and Love's short practice of surgery, 18th edition, 1982)
There is rich lymphatic supply, the lymphatic plexus draining into the tracheal, deep cervical and mediastinal lymph nodes.
The nerves supplying the thyroid gland arise from the sympathetic trunk (mainly from the middle cervical ganglion, partly from the superior and inferior ganglion), and the vagus nerve (through superior and inferior laryngeal nerves and possibly from glossopharyngeal nerve).
The thyroid also contain the parafollicular or c-cells that secrete calcitonin, the calcium lowering hormone. Embryologically, these develop from the last pair of pharyngeal pouches but settle amongst the cells of follicular epithelium or in the thyroid interstitium. C-cell hyperplasia is seen in familial medullary carcinoma of thyroid.
The parathyroid glands are closely applied to the thyroid gland, but lie outside its capsule. The upper pair are fairly constant and situated on the posterolateral border, above the point of entry of the inferior thyroid artery. The lower pole is usually situated at the lower poles, but their position may be variable and they may be found anywhere along a line extending from the lower pole to the upper pole of thymus (Fig. 1.2). The importance of correctly identifying their position lies in preserving them during thyroidectomy.
EMBRYOLOGICAL DEVELOPMENT
The thyroid gland develops from the first branchial pouch behind the unpaired germ of the tongue-embryologically, it is part of the alimentary canal and these share the thyroid gland's ability to concentrate iodide. The foramen cecum in the tongue marks the point of outgrowth of the gland. The epithelial cord growing from this point merges with the thyroglossal duct in its upper position before developing into the two lobes.
Fig. 1.2: Anatomical relations of the parathyroid gland(from Bailey and Love's short practice of surgery, 18th edition, 1982)
At the end of the 4th week of intrauterine life, the thyroglossal duct atrophies leaving only the foramen cecum of the tongue. The pyramidal lobe is a remnant of the thyroglossal duct.
PHYSIOLOGY OF THYROID HORMONE SECRETION
The fetal thyroid starts developing at about 4 weeks and a small amount of fetal thyroid hormone formation starts. From 20th to 22nd week, fetal TSH secretion, together with T3 and T4 secretion, rise steadily to help fetal CNS development and maturation.
The formation and secretion of thyroid hormone involves several steps of iodine metabolism and trapping, its oxidation and iodothyronine formation. The thyroid hormones are stored in the thyroid follicles as thyroglobulin. Once released in the circulation, they are bound to Thyroxine Binding Globulin (TBG) and Thyroid Binding Pre-Albumin (TBPA).6
The steps in formation and release of thyroid hormones are as follows:
- Iodine trapping: This is against a concentration gradient, since plasma concentration of Iodide is low. It is through the action of Sodium Iodide Symporter system (NIS).
- Iodide oxidation and organification: Iodide is oxidized and organified to Monoiodotyrosine (MIT) and Diiodotyrosine (DIT) through the action of thyroid peroxidase (TPO). The reactions occur within the thyroglobulin.
- Iodothyronine synthesis: In presence of TPO, two DIT molecules fuse to form Tetraiodothyronine or T4, one MIT and DIT fuse to form Triiodothyronine. However, the main source of T3 is through the peripheral conversion of T4 (Flow chart 1.1).
- Storage and release of thyroid hormones: The process of pinocytosis through pseudopodia formation at the apical area of the thyroglobulin leads to release of the thyroid hormone in the general circulation.
- Peripheral conversion: 80% of T3 is through the peripheral conversion of T4, through deiodination.
Both T3 and T4 are metabolized in several ways. Mainly, they are deiodinated, but partly, they are conjugated in the liver. T4 conjugates with glucoronic acid and T3 with sulphate esters. The conjugates are secreted in bile and partly reabsorbed or excreted through the gastrointestinal tract.
The thyroid hormones have an action on all metabolic activities of the body. This includes carbohydrate, protein, lipid, mucopolysaccharide metabolism. Clinical signs and symptoms of thyroid hormone derangement therefore lead to abnormal glucose metabolism, production of mucopolysaccharide (hyaluronic acid) that causes water retention and increased or decreased cholesterol.