Structure of Thyroid Gland:
The thyroid gland is situated in the neck in front of the larynx and trachea at the level of the 5th, 6th, and 7th cervical and 1st thoracic vertebrae. It is a highly vascular gland weighing about 25 g and is enclosed within a fibrous capsule.
- It resembles a butterfly in shape, consisting of two lobes on either side of the thyroid cartilage and upper cartilaginous rings of the trachea, joined by a narrow isthmus.
- Each lobe is cone-shaped, about 5 cm long and 3 cm wide.
- Arterial blood supply comes from the superior and inferior thyroid arteries — branches of the external carotid and subclavian arteries, respectively.
- Venous return occurs through the thyroid veins, draining into the internal jugular veins.
- The recurrent laryngeal nerves pass close to the lobes, particularly on the right side, near the inferior thyroid artery.
- The gland is composed of spherical follicles lined by cuboidal epithelium, which secrete and store colloid. Between follicles lie parafollicular (C) cells that secrete the hormone calcitonin.
Thyroxine (T4) and Tri-iodothyronine (T3):
Iodine is essential for the synthesis of thyroid hormones — thyroxine (T4) and tri-iodothyronine (T3). These contain four and three atoms of iodine, respectively. Main dietary sources include seafood, iodized salt, and vegetables grown in iodine-rich soil.
The thyroid gland traps iodine from the blood to form thyroglobulin, the precursor for T3 and T4. Their release is stimulated by thyroid-stimulating hormone (TSH) from the anterior pituitary, which in turn is regulated by thyrotropin-releasing hormone (TRH) from the hypothalamus.
Secretion of T3 and T4 is controlled by a negative feedback mechanism. Increased levels of these hormones suppress TSH and TRH secretion, whereas decreased levels have the opposite effect.
Functions of Thyroid Hormones:
- Increase the basal metabolic rate and body heat production.
- Regulate carbohydrate, protein, and fat metabolism.
- Promote growth and development, especially of the skeleton and nervous system.
- Enhance the effects of adrenaline and noradrenaline.
T3 is more potent and physiologically active than T4, though T4 is produced in greater quantity and converted into T3 within target tissues.
Calcitonin:
Calcitonin is secreted by the parafollicular or C-cells of the thyroid gland. It lowers elevated blood calcium (Ca2+) levels by:
- Promoting calcium storage in bones.
- Inhibiting calcium reabsorption from renal tubules.
Calcitonin’s effects are opposite to those of parathyroid hormone (PTH). Its release is stimulated by increased calcium levels and is most important during childhood when bone remodeling is rapid.
Parathyroid Glands:
There are four small parathyroid glands, each about 50 mg in weight, located on the posterior surface of the thyroid gland — two in each lobe. They are surrounded by a thin connective tissue capsule and contain columnar cells with blood-filled sinusoids between them.
Function of Parathyroid Hormone (PTH):
- PTH secretion is regulated by blood calcium levels — low calcium stimulates its release.
- PTH increases blood calcium levels by:
- Enhancing calcium absorption from the intestine,
- Increasing calcium reabsorption in renal tubules, and
- Stimulating bone-resorbing cells (osteoclasts) to release calcium into the blood.
Together, parathormone and calcitonin maintain normal blood calcium levels — essential for muscle contraction, nerve impulse transmission, blood clotting, and enzyme function.
Detailed Notes:
For PDF style full-color notes, open the complete study material below: