Present in anterior and lateral surface of trachea
Present at 5th to 7th cervical vertebrae and and 1st thoracic vertebrae
3 of 23
4 of 23
There are two lobes in the thyroid gland
Covered with fibrous tissue
A mass of tissue is called isthmus
Lobes are pyramidal shape
They may be extended towards isthmus
5 of 23
Thyroid tissue is composed of tiny structural units called follicles
Each follicle is made up of simple cuboidal glandular epithelium
This is filled with a thick fluid called thyroid colloid
The colloid is produced by follicular cells, para follicular cellular present between follicular cells helps in formation of calcitonin hormone
6 of 23
7 of 23
8 of 23
9 of 23
Thyroxine and triiodothyronine
Thyroxine T4
Tri-iodothyronine T3
T4 is most abundant
10 of 23
T3 and T4
Iodine is essential for formation on T3 and T4
The bodyβs main dietary sources of iodine are seafood, vegetables grown in iodine-rich soil and iodinated table salt.
thyroid gland selectively takes up iodine from the blood, a process called iodine trapping.
11 of 23
Release of T3 and T4 into the blood is stimulated by thyroid stimulating hormone (TSH) from the anterior pituitary.
Secretion of TSH is stimulated by thyrotrophin releasing hormone (TRH) from the hypothalamus.
secretion of TRH is stimulated by exercise, stress, malnutrition, low plasma glucose levels and sleep.
12 of 23
Goiter
When the supply of iodine is deficient, excess TSH is secreted and there is proliferation of thyroid gland cells and enlargement of the gland
Secretion of T3 and T4 begins about the third month of fetal life and is increased at puberty and in women during the reproductive years, especially during pregnancy
13 of 23
14 of 23
Hyperthyroidism is an overactive thyroid (when it produces too much thyroid hormone).
Hypothyroidism is an underactive thyroidΒ (when it does not produce enough).
15 of 23
16 of 23
Calcitonin
Hormone is secreted by the parafollicular or C-cells in the thyroid gland
It acts on bone cells and the kidneys to reduce blood calcium (Ca2+) levels when they are raised
It promotes storage of calcium in bones(osteoblast) and inhibits reabsorption of calcium (osteoclast) by the renal tubules.
17 of 23
Blood Supply
Blood supply to this gland is high
superior thyroid artery (STA) arising from the external carotid artery (ECA),
and the inferior thyroid artery (ITA) branching from the thyrocervical trunk
Venous return is through thyroid veins
Nerve Supply
Superior, middle, and inferior cervical sympathetic ganglia
18 of 23
Histology
Thyroid is covered by a thin capsule of dense irregular connective tissue
From the capsule fine septa pass and divide into indistinct lobules,
these septa conduit for blood vessels, lymphatic vessels and nerve fibers
Parenchyma of thyroid gland consists of 20-30 million spherical cyst like structure called follicles (0.2 to 1.0 mm in diameter)
Wall of follicles lined by simple cuboidal epithelium
19 of 23
The cavity of follicle contains glue-like material called colloid
Lining epithelium of thyroid follicle contains two types of cells
Follicular cells (principal or thyroid cells generally cuboidal in shape)
They depend upon functional activity of the gland
In active gland cells are low cuboidal
In hyperactive gland they assume columnar shape
20 of 23
Parafollicular Cells
Lie singly or in small groups among follicular cells but do not extend to the lumen follicles
Lie in basal lamina of thyroid follicles
Larger than follicular cells but their cytoplasm stains poorly called c-cells clear cells
Electron microscope shows parathyroid glands shows a round nucleus, RER, mitochondria, well developed Golgi Apparatus.