thyroid gland

Thyroid gland 

Thyroid gland is butterfly shaped is one of the largest pure endocrine glands in body.

Location

It is located in anterior neck, just inferior to larynx.

It has two pyramidal shaped lobes with one on either side of trachea at level of 5th, 6th and 7th cervical and 1st thoracic vertebrae.

Weight 

Each lobe is 5cm long and 3cm wide. It is highly vasular gland that weights about 25-30 gm and surrounded by fibrous capsule.

Its two lateral lobe are connected by median tissue mass called isthmus anterior to trachea.

Blood supply

Arterial blood supply to thyroid gland is through superior and inferior thyroid artery.

Venous return is by thyroid vein, which drain into internal jugular veins.

Histology of thyroid glands

The thyroid gland is composed of hollow, spherical sacs called follicles.

The wall of follicle are formed by cuboidal epithelium cells called follicular cells.

Central cavity or Leumen of follicles, secrete and store is colloid, a thick sticky protein material consisting of thyroglobulin molecules with attached iodine atoms.

Thyroglobulin stores large amount of thyroid hormones.

Between the follicles there are other cells found; parafollicular cells or C cells.

These cells found in clusters between thyroid follicles.

C cells, which secretes hormone calcitonin.

Hormone of thyroid gland

Thyroid hormone

Follicular cells of thyroid gland secretes two iodine containing hormone. 

Iodine is essential for formation of thyroid hormone.

1. T3 Tri-iodothyronine (containing three iodine atoms)

2. T4 Thyroxine or Tetraiodothyronine ( containing of four iodine atoms)

3. Calcitonin

The release of T3 and T4 into blood is stimulated by thyroid stimulating hormone (TSH) from anterior pituitary.

Thyroid gland also produces and releases hormone calcitonin, which contribute regulation of blood calcium levels.

Calcitonin decreases concentration of calcium in blood.

Most calcium removed from blood is stored in bones.

Synthesis of thyroid hormones

Thyroid hormone synthesis involve six process that begin when thyroid stimulating hormone (TSH) secreted by anterior pituitary to binds to follicle cell receptors.

1. Formation and storage of thyroglobulin; follicle lumen stored colloid.

2. Iodide trapping ; thyroid gland selectively takes up iodine from blood, process called iodine trapping.

3. Oxidation of iodide and iodine; converted iodide to iodine

4. Iodination of tyrosine

5. Coupling of T2 and T1

6. Colloid endocytosis

7. Cleavage /separation of hormone for release into blood stream.

Regulation of Secretion

Secretion of thyroid stimulating hormone (TSH) is stimulated by thyrotropin releasing hormone (TRH) from hypothalamus and secretion of thyrotropin releasing hormone (TRH) is stimulated by; exercise, stress, malnutrition, low plasma glucose level, and sleep.

Level of secretion of thyroid stimulating hormone (TSH) depends on plasma cells of T3 and T4.

Through negative feedback mechanism, increases level of T3 and T4 decrease thyroid stimulating hormone (TSH) secretion and vice versa.

Excercise, stress, malnutrition, low blood glucose, sleep

Hypothalamus

Thyroid releasing hormone (TRH)

Anterior lobe of pituitary gland

Thyroid stimulating hormone (TSH)

Thyroid gland

T4, T3

Raise blood level of T3 and T4

Use of Hormones by most body cell

Lowered levels of T3 and T4

(Negative feedback regulation of Secretion of T4 and T3).

Significance of Iodine

Thyroid hormone cannot produced without an abundant source of Iodine.

When supply of iodine is dificiency, body will try excess TSH is secreted.

Which result in swelling and enlargement of thyroid gland, resulting goiter.

Function of Thyroid Hormone (T3 and T4)

1. Primary effect of TH is to increase body's metabolic rate.

2. Increasing basal metabolic rate and heat production.

3. Regulating metabolism of;

Carbohydrates (CHO)

Proteins

Fats

4. Thyroid hormone promote alertness, bone growth and remodeling and development of; 

Skin, Hair, Nails, Teeth, Fetal nervous system, muscular system, skeletal development.

5. It also stimulates pituitary gland to secrete growth hormone.

6. It promotes normal functioning of heart.

Promotes normal gastrointestinal motility and increase secretion of digestive juice.

Abnormal secretion of thyroid hormone.

1. Hypothyroidism

(decrease T3 and T4 secretion)

Goiter disease

Bradycardia

Hypothermia

Constipation

2. Hyperthyroidism

(Increase T3 and T4 secretion)

Gravis disease

Trachycardia

Heat intolerance

Diarrhea

Calcitonin

Calcitonin hormone is secreted by C- cell in thyroid gland.

It contribute to regulation of blood calcium lever.

It act on bone cells and kidney to reduce blood calcium levels when they are raised.

Its most important effect is to lower blood Ca2+ levels.

It promotes storage of calcium in bones and inhibits reabsorption of calcium by renal tubules.

It's effect is opposite to that of parathyroid glands.

Release of calcitonin is stimulated by increase in blood calcium levels.

This hormone is important during childhood when bones undergo considerable changes in size and shape.

Calcitonin can also be used therapeutically for treatment of Hypercalcemia or osteoporosis.