Introduction

Thyroid is the largest endocrine gland (5×3cm), weighing 25gm in a healthy adult. It lies on the anterolateral side of the cervical trachea, extending from the level of the 5th cervical vertebra to the 1st thoracic vertebra. Thyroid gland is brownish red in color.

Structure

Thyroid gland comprises of two lobes (bilobed) interconnected by a transverse glandular band (isthmus), and appearing as a butterfly. It is therefore described as an H-shaped organ.

From isthmus, extends a small, pyramidal-shaped lobe in the upward direction. Around 3 million of small, oval, or rounded thyroid follicles and stromal tissues make up the histological structure of thyroid gland. These follicles, inter-linked via connective tissue are thyroid disease bounded by a white fibrous capsule. The follicles lined by cuboidal glandular follicular epithelium of aclinal cells surround colloid (a gelatinous material which is an inactive glycoprotein thyroglobulin). The connective tissue and the spaces between thyroid follicles have groups graves disease of scattered parafollicular cells (endocrine cells).

Thyroid gland is immensely vascularized and receives 80-120ml of blood per minute immune system.

Hormones

Thyroid stimulating hormone released by anterior pituitary gland regulates the secretion of thyroxine and Thyrocalcitonin (TCT) hormones from the thyroid gland.

Thyroxine

Thyroxine is an iodine-containing amine hormone. It is made up of 65% of iodine and is tyrosine-derived. It exists as tetra-iodothyronine, and also sometimes as triiodothyronine.

Functions

1) Thyroxine regulates the body's Basal Metabolic Rate (BMR).

2) It controls urine output by regulating the working of renal tubules.

3) It helps in homeothermy in warm blooded animals.

4) It stimulates protein synthesis.

5) It regulates the development of mental faculties.

6) It increases the action of adrenaline and nor-adrenaline hormones.

Thyrocalcitonin (TCT)

Thyrocalcitonin is a long peptide hormone which is released by the parafollicular cells of thyroid gland. Its secretion is controlled via a feedback mechanism of increased levels of calcium in plasma. Since TCT is a hypocalcemia factor, it normalizes the blood calcium level by:

1) Increasing deposition of calcium in the bones, thus checking osteoporosis, and

2) Decreasing reabsorption of calcium from urine, thus increasing a Ion excretion to prevent hypercalcemia.

Functions

l) TCT along with the parathyroid hormone regulates the metabolism of calcium and phosphate.

2) It provides protection to the organism from the consequences of increased blood calcium levels.

3) It inhibits bone resorption, thus reducing blood plasma concentration of calcium and phosphate; this is because these minerals are the major constituents levels of thyroid hormone of bone mineral and their ionic forms are released in the blood when bone is resorbed.

4) It also increases phosphate excretion in the urine in some case; however, it is still not clear whether it is a blood test, a direct effect of TCT on kidneys or an indirect effect due to the declining blood calcium level (occurring when bone resorption is inhibited).

Functions

Thyroid gland performs the following functions:

1) It controls the body's metabolic activities (the body's ability of converting food into energy).

2) It secretes hormones which regulate thyroid cancer, vital organs and maintain the internal homeostasis.

3) It controls the breathing and heart rate.

4) It monitors the body weight, thus defective thyroid gland in an individual results in severe weight variation.

5) Its wings or lobes produce thyroid hormone.

6) It secretes a hormone which controls the internal body temperature and cholesterol levels.

7) The hormones it secretes also increase cellular metabolic activity, thus influencing the metabolic rate and iodine deficiency protein synthesis, which in turn facilitates normal development (since development relies on protein synthesis).

2) PARATHYROID GLAND

Introduction

Parathyroid glands are small endocrine glands located in the neck and produce parathyroid hormone. Four parathyroid glands are positioned behind the thyroid gland, or within the thyroid gland (in rare cases) or in the chest. These glands regulate the

Structure

Four pea-shaped parathyroid glands are either completely or partially embedded in the dorsal surface of thyroid function gland (figure 7.6); each lobe of thyroid has two oval-shaped' small sized (5 x 5mm) and yellow-coloured parathyroid glands.

Masses of polygonal cells (known as chief and oxyphil cells) arranged in cords make up the histological structure of a parathyroid gland. Oxyphil cells are larger' found singly or in small groups, and are less primary hyperparathyroidism numerous thyroid surgery than the chief cells.

Hormone

Parathyroid Hormone (PTH), also called Collop’s hormone increases the Ca2+ ion levels in blood. It acts on bones and stimulates dissolution or de-mineralisation of their calcium.

It is thus clear that PTH is antagonistic to TCT hormone and is hypercalcemia, i.e., it increases the blood Ca2+ ion levels. Thus both, PTH and TCT together play a significant role in maintaining calcium balance in the body. PTH increases the calcium level by stimulating the following processes:

1) Mobilisation of Calcium from Bone: PTH increases the release of calcium in blood by stimulating the osteoclasts to reabsorb bone mineral.

2) Enhancing Absorption of Calcium from Small Intestine: PTH increases blood calcium level by assisting the small intestine to absorb calcium. This process is indirectly stimulated by PTH as it triggers the production of vitamin D active form in the kidney. Vitamin D in turn facilitates the synthesis of a calcium-binding protein in the epithelial cells of intestines, and this protein allows calcium absorption into blood.

3) Suppression of Calcium Loss in Urine: PTH maintains blood calcium level by stimulating its fluxes into the blood from bone and intestine, and limiting its excretion via urine (facilitated by stimulating radioactive iodine tubular reabsorption of calcium). PTH also stimulates phosphate ions excretion via urine.

Functions

Parathyroid gland produces and releases PTH to maintain the blood calcium level (whenever its level declines) by:

1) Breakdown of bone to release calcium, as bone stores maximum amount of calcium,

2) Facilitating calcium absorption from food, and

3) Limiting calcium loss via urine.

The gland stops releasing the hormone when the calcium level reaches normal or becomes high. Calcium should be maintained at an adequate level since it is essential for the normal functioning of heart, nervous system, kidneys, and bones thyroid stimulating hormone tsh.

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