Structure Of a Normal Thyroid Gland and Its Endocrine Functions
Coverings of The Thyroid
A mature thyroid is ensheathed by a true capsule that is the peripheral condensation of the connective tissue of the gland, and a false capsule that is derived from the pretracheal layer of deep cervical fascia.
A dense network of blood capillaries lies beneath the true capsule. To avoid hemorrhage during operations, the thyroid is removed along with its true capsule.
Appearance, Location and Components of a Normal Thyroid Gland
Thyroid gland is a brownish-red, butterfly shaped, highly vascular structure present in the lower part of the neck. The word thyroid literally means, "shield like", so named due to the shape of this gland.
It consists of two lobes - right and left, that are connected by a narrow median isthmus. Each lobe is conical in shape with its apex diverging literally to the level of the oblique line on the lamina of thyroid cartilage, while the base resting on the fourth or the fifth tracheal cartilage.
Each lobe is about 5 cm long, attached from behind to the cricoid cartilage, bound from the sides by the sternothyroid muscle. In the front each lobe is covered by the sternohyoid, superior belly of omohyoid and anterior border of sternocleidomastoid muscles. The median surface of each lobe is adapted to the larynx and trachea or the wind pipe.
The isthmus measures about 1.25 cm transversely and vertically and connects the lower parts of both lobes together and sits on the second and third tracheal cartilages.
A conical pyramidal lobe often ascends towards the hyoid bone from the isthmus or adjacent part of either lobe (more often the left). Sometimes a fibrous or fibro muscular band, the levator of thyroid gland descends from the body of the hyoid bone to the isthumus or pyramidal lobe.
Small detached masses of thyroid may occur above the lobes or isthmus as accessory thyroid glands.
Vestiges of the thyroglossal duct, an embryonic structure from which the gland has been developed, may persist as accessory nodules or thyroid cysts near the mid line or even in the tongue.
A normal thyroid gland weighs around 25 gms on an average. It is slightly heavier in females, and enlarges during menstruation and pregnancy.
Structure of A Normal Thyroid Gland
Blood Supply and Innervation of Thyroid Gland
Thyroid gland receives its blood supply from superior and inferior thyroid arteries. Superior thyroid artery supplies the upper one-thirds of each lobe and upper half of isthmus, while the inferior thyroid artery supplies lower two-thirds of each lobe and lower half of isthmus. However, the superior and inferior thyroid arteries anastamose freely both on the surface of the gland and in its substance and the territories supplied by them overlap considerably.
Sometimes in 3% of individuals, the thyroid is also supplied by the lowest thyroid artery or thyroidea ima artery which arises directly from the arch of aorta.
Accessory thyroid arteries arising from tracheal and esophageal arteries also supply the thyroid.
Thyroid is drained by the superior, middle and inferior thyroid veins.
Lymph from the gland drains into deep cervical lymph nodes directly and through pretracheal and paratracheal nodes.
It receives its nerve supply from superior, middle and inferior cervical ganglia.
Histopathology of Thyroid Follicles
Thyroid is covered by a thin capsule of connective tissue from which extend the septae that divide the gland into multiple lobules.
Each lobule is made up of an aggregation of spherical follicles that consist of masses of colloid bound by a layer of cells that rest on a basement membrane.
The colloid contains an iodinated glyco protein, iodothyroglobulin, the precursor of thyroid hormones tri-iodothyronine (T3) and tetra-iodothyronine or thyroxine (T4) and a product of the follicular epithelial cells.
The space between the follicles is filled by a stroma made up of delicate connective tissue in which there are numerous capillaries and lymphatics.
Follicular cells that line the colloid masses vary from squamous to columnar in shape depending on their activity that is mainly controlled by circulating TSH (thyrotropin) hormone produced by the anterior lobe of pituitary gland. In the absence of TSH, the follicular cells are flattened or squamous and in resting phase with the lumen colloid being abundant and reflecting increased storage of iodinated thyroglobulin. With the secretion of TSH from anterior pituitary, these follicular cells get activated and begin to engulf colloid droplets at the lumen aspects, become columnar in shape and start processing the colloid to produce thyroid hormones.
Prolonged high levels of circulating TSH induce follicular hypertrophy and hyperplasia (increase in size and number of thyroid follicles) with progressive resorption of colloid and increased vascular supply of thyroid stroma.
Iodine that is an essential constituent of thyroid hormones is taken up from bloodstream through the blood capillaries that surround the thyroid follicles.
Synthesis of Thyroid Hormones
The synthesis and release of thyroid hormones takes place in two phases.
In the first phase thyroglobulin is synthesized and is packed into secretory vacuoles in the follicular cells. The vacuoles travel to the lumen surface where they release thyroglobulin into the follicular cavity. Here the thyroglobulin combines with iodine to form mono iodotyrosyls and di iodo tyrosyls which in turn form the colloid. Colloid is iodinated thyroglobulin.
In the second phase, particles of colloid are taken back into the follicular cells. Within the cells, the iodinated thyroglobulin is acted upon by the enzyme peroxidase, releasing the hormones T3 and T4 which pass through the cells and are released in blood.
Control of Thyroid Hormones and Their Role in Normal Functioning of Our Body
Function of Thyroid Hormones
The major functions of thyroid hormones include the following :
- Increased Basal Metabolic Rate and calorigenesis.
- Growth and development.
- Carbohydrate metabolism and utilization of sugars.
- Protein and fat degradation.
Of the two thyroid hormones, tri iodothyronine or T3 is the main stimulator of cellular metabolic rate, its action being very strong and immediate, whereas tetra iodothyronine or T4 is powerful but its onset of action is less rapid.
Overproduction of both thyroid hormones causes thyrotoxicosis (exopthalmic goitre); while reduced secretion in adults produces myxedema and in infants cretinism.
Their synthesis and release is mainly controlled by TSH, thyroid stimulating hormone from anterior pituitary.
In Grave's disease, antibodies to human thyroid stimulating immunoglobulin bind to TSH receptor sites on the follicular cells interfering with this control and cause excess hormone production.
Any enlargement of the thyroid is called goitre.
Thyroid Parafollicular Cells
Thyroid Parafollicular Cells, Calcitonin Secretion and Function
Thyroid parenchyma also consists of another cell type, the parafollicular cells, also called the light cells, clear cells or C-cells, producing the peptide hormone Thyrocalcitonin.
C-cells, an APUD element (Amine Precursor Uptake and Decarboxylation) are derived from the ultimobranchial bodies. They are polyhedral in shape and lie between the follicular cells and their basement membrane. They may lie between the adjoining follicular cells; but they do not reach the lumen.
Parafollicular cells secrete the hormone thyrocalcitonin that has an action opposite to that of the parathyroid hormone on calcium metabolism. It comes into play when serum calcium levels are high. It tends to lower the calcium levels by suppressing the release of calcium from the bones. It does so by inhibiting bone resorption by osteoclasts.
Various hormones produced by Thyroid Gland
T3 and T4
Control metabolic rate
Mode of action
Increase heart rate, utilization of sugars and protein degradation
Suppress bone resorption
Thyroid - an Endocrine Unit
Thyroid is an endocrine gland situated in the lower part of front of neck. It regulates Basal Metabolic Rate, stimulates bodily and psychic growth and plays an important role in calcium metabolism.