- It is situated in the neck in front of the larynx and trachea at the level of 5th, 6th, and 7th cervical and 1st thoracic vertebrae.
- It is highly vascular ‘H’ shaped brownish red gland.
- It weighs about 25 grams which is surrounded by fibrous capsule.
- It consists of 2 lobes, one on either side of the thyroid cartilage and upper cartilaginous rings of the trachea.
- The lobes are joined by a narrow isthmus, lying in front of the trachea.
- Each lobe is cone shaped, about 5 cm long and 3 cm wide.
- It is one of the largest endocrine gland.
- Arterial supply is through the superior and inferior thyroid arteries.
- Superior thyroid artery is the branch of the external carotid artery and the interior thyroid artery is the branch of the sub-clavian artery.
- Venous drainage is by thyroid veins, which drain into the internal jugular veins.
- The gland is composed of thyroid follicles.
- Each follicle is lined by cuboidal epithelium and filled up by the colloids.
- In the interval between the follicles are occupied by the stroma and parafollicular cells or C- cells.
- Thyroid hormones (triiodothyronine (T3) and tetraiodothyronine (T4 or thyroxine) are produced by these follicular cells.
- Iodine is necessary for the formation of the thyroid hormones.
- Parafollicular cells secrete calcitonin which reduces Ca++ in body fluids when Ca++ levels are elevated.
Regulation of thyroid hormones
- Stumuli such as stress, hypothermia causes release of thyroid releasing hormone (TRH) from the hypothalamus.
- TRH from hypothalamus causes the release of thyroid releasing hormone (TSH) from anterior pituitary which causes secretion and storage of thyroid hormones (T3 and T4) from and within the thyroid gland.
- Increasing levels of thyroid hormones has negative feedback i.e. T3 and T4 inhibit the TRH and TSH secretion.
Functions of thyroid hormones
- Increases the basal metabolic rate (BMR) by increasing the oxygen consumption of the tissues.
- Increases the synthesis of protein in the cells.
- Increases the heat production in the body.
- Increases the formation of many enzymes.
- Increases the overall activity of cardiovascular system.
- Increases the rate and force of respiration.
- Increases the appetite and food intake.
- Decrease the fat storage by mobilizing it from adipose tissues and fat droplets.
- Decreases the plasma cholesterol level by increasing its excretion from liver cells into bile.
- Plays role in metabolism of carbohydrates.
- Promote growth and development of the brain during the fetal life and the first few years of postnatal life.
- Important for the development and maintenance of normal functioning of central nervous system.
- Essential for normal activity of skeletal muscles.
- Has role in sexual functioning.