3) Thyroid Gland Mdsc 2101 2014

THE THYROID GLANDThe thyroid gland is the lobulated, butterfly

-shaped and highly vascular endocrine glandlocated in the cervical region where it clasps the upper part of the trachea.2.It spans vertebral C5 to T1Reddish-brown in appearance and weighing

about 25gm, it is made up of two triangular-shaped lobes which are connected across themidline by a narrow part referred to as theisthmus (See Figure 1).

Each lobe measures about 5cm rostrocaudally and has three surfaces (anterolateral, posterior and medial) an upper pole (Its apex) and a lower pole (The base). Furthermore, each lobe extent rostrocaudally from the oblique line of the thyroid cartilage to the sixth tracheal ring (Cartilage). The entire gland is enclosed in a fascial sheath

(The pretracheal fascia), which attaches the gland superiorly to the oblique line of the thyroid cartilage and the arch of the cricoids cartilage.

This attachment is responsible for the movement of the gland in the process of swallowing and phonation. This is of clinical significance in distinguishing a thyroid mass from other cervical masses. The latter will not move during swallowing and phonation.

Relations of the Thyroid GlandThe concave medial surface of each lobe is related to:Upper part of the tracheaUpper part of the esophagusThe recurrent laryngeal nerve lies between the trachea and esophagusCricoids cartilageThyroid cartilageCricothyroid muscleInferior constrictor muscleExternal branch of the superior laryngeal nerveArteries and venous network (anastomoses) of the thyroid gland

The posterior surface of each lobe is related to:The parathyroid glands are embedded in this surfacePrevertebral fasciaLongus colli muscleThe carotid sheath which encloses the common carotid artery, the vagus nerve and the internal jugular vein.

The convex anterolateral surface of each lobe is related to:Sternothyroid muscleSternohyoid muscleOmohyoid muscle Superior belly)Medial part of the sternocleidomastoid muscle

The narrow isthmus lies anterior to the second to the fourth tracheal cartilage. It is cover anteriorly by the cervical fascia and skin. A pyramidal lobe of the gland may be attached to the left aspect of the upper border of the isthmus. This lobe is usually attached to the hyoid bone by a muscular or fibrous band referred to as the Levator Thyroideae Glandulae.

Development of the Thyroid GlandThe thyroid is the caudal expansion of the Thyroglossal duct, an epithelial down growth from the junction of the first and second pharyngeal arches (Foramen caecum). The duct runs anterior to the hyoid bone, the thyroid and cricoids cartilages. It is firmly attached to the hyoid bone and the segment between the hyoid bone and the isthmus becomes the definitive Levator Glandulae Thyroideae.

Blood Supply and Venous Drainage of the Thyroid glandThe thyroid gland is irrigated by (Fig: 2): The inferior thyroid artery, the largest branch of

the thyrocervical trunk of the subclavian artery. Branches of this artery approach the gland from its lower pole and supplies several branches to the posteroinferior aspects of the glandThe superior thyroid artery, which is the first

branch of the external carotid artery. It approaches the gland from its superior pole and through anterior and posterior branches supplies the anterosuperior aspects of the gland.

The Thyroidae ima artery is the third artery

which supplies the gland but only in 10% of the population. It is a slender branch that could have as its parent trunk, the brachiocephalic trunk, the arch of the aorta, the right common carotid, right subclavian or the right internal thoracic arteries. This artery runs upward anterior to the trachea

towards the isthmus of the gland. It supplies the isthmus as well as the trachea. The location of this artery is of clinical

importance as it could be inadvertently slit in the course of a tracheostomy surgical procedure.

Venous Drainage:The thyroid gland is drained by three pairs of

veins. These are:The superior thyroid vein a companion vein of

the superior thyroid artery drains the superior part of the gland into the internal jugular veinThe middle thyroid vein which runs parallel to

the inferior thyroid artery drains the middle part of the gland into the internal jugular veinsThe inferior thyroid vein drains the inferior part

of the gland into the brachiocephalic veinsThe three pairs of vein form a thyroid venous

plexus anterior to the gland and the trachea.

Lymphatic Drainage of the Thyroid GlandLymph vessels are widely distributed in the interlobular connective tissue from which lymph is drained superficially to a capsular network of lymph vessels.From the network, lymph is drained into three groups of lymph nodes. These arePrelaryngeal nodes from which lymph drains into superior deep cervical nodesPretracheal nodes from which lymph drains into inferior deep cervical lymph nodesParatracheal lymph nodes from which lymph drains into inferior deep cervical lymph nodesSome lymph vessels from the gland may drain directly into the Brachiocephalic lymph nodes or the thoracic duct.

Innervation of the Thyroid GlandThe thyroid gland derives its vasomotor fibres

from the superior, middle and inferior cervical sympathetic ganglia. These fibres accompany the arteries of the

gland. The gland is devoid of secretomotor fibres since

the secretion of its hormone is controlled by hormones of the pituitary gland.

Microscopic Anatomy of the Thyroid Gland (Figs: 3 & 4)The thyroid gland is invested in two coats of connective tissue; an outer pretracheal fascial sheath and an inner thin fibrous capsule. The inner capsule sends into the substance of the gland connective fibres which form the interlobular septa of the thyroid gland. Embedded in the septa are the blood/lymphatic vessels and nerves within the substance of the gland.

The lobules between the septa consist of spheroidal-shaped follicles filled with a homogeneous colloid material of variable sizes.

The follicles are enclosed by a simple cuboidal epithelial cell layer which is supported by a basal lamina. Furthermore, the follicles are filled with Thyroglobulin which is the storage form of Thyroxine (T4) and Tri-iodothyronine (T3). About 90% of thyroid hormone is in the form of T4 but the active form of the hormone is T3

(Thyroglobulin is Glycoprotein conjugated to iodine).

Synthesis, Storage and Release of Thyroid hormones

An active, hormone-synthesizing/secreting gland is characterized by the conversion of cuboidal epithelial cell to tall columnar epithelial cells.A second types of cells (Thyroid C cells or Parafollicular cell C) are also encountered in the thyroid gland. These are often found singly intervening amongst the epithelial cells or in clusters in the septa between the follicles. They are usually larger than the epithelial cell but pale staining with abundant granular cytoplasm. These cells secrete Calcitonin which antagonizes Parathormone of the parathyroid gland by suppressing the osteoclastic resorption of bone. This action results in the lowering of blood calcium level.

CLINICAL CORRELATESTHYROTOXICOSIS: There are two forms of the disease:1Nodular Hyperplasia: A small number of follicular cells may undergo hyperplasia leading to continuous synthesis and secretion of thyroid home without a resting, inactive phase. This is referred to as Nodular Hyperplasia of the thyroid gland. This might present as an isolated nodule in the gland.

2.Graves disease (Diffuse Hyperplasia):This is characterized by the production of auto antibodies to TSH receptors, which act as Long-Acting Thyroid Stimulator (LATS) which continuously stimulates the entire follicles leading to excessive secretion of thyroid hormone (Hyperthyroidism) and enlargement of the gland (Goiter)Graves disease is also accompanied by inflammation and growth of extraocular adipose tissue leading to protrusion of the eyeball (exophthalmos)

In about 50% of cases, patients present with exophthalmos (Exophthalmic Graves disease)

HYPOTHYROIDISM: There are various forms of Hypothyroidism which includes:Iodine Deficiency Goiter in which there is

inadequate consumption of iodine leading to Impaired secretion of thyroid hormone leading to Excessive secretion of thyroid stimulating

hormone (TSH) leading to Enlargement of the thyroid gland (Goiter).

If this occurs from birth, it might lead to cretinism (Hypothyroid dwarfism or Infantile Hypothyroidism)Hashimotos disease is an autoimmune disease of

the thyroid gland characterized by impairment of thyroid function. Antibodies against thyroid tissue are detected in the blood stream of this patients

Developmental Anomalies of the Thyroid Gland:An Ectopic thyroid gland may be found within the

substance of the tongue (Lingual Thyroid) orAt any point along the path of descent of the

Thyroglossal duct orIn the thymus gland, On the thyrohyoid muscle or In the thorax (Accessory thyroid gland)Sometimes, the isthmus is absent (See Figure)