Thyroid Gland 1 B.Pharmacy 1st Semester 2023 (2022-2023) Previous Year's Question Papers/Notes Download - HK Technical PGIMS

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THYROID GLAND

 Prof. Preety Sinha
 Deptt. of Zoology
The thyroid gland is the largest gland located in the neck at about the level of the larynx
and upper part of the trachea. . It consists of two lobes connected by a narrow bridge
(isthumus) across the ventral surface of the trachea. The gland is well supplied with blood,
has a connective tissue framework and contains many microscopic cavities or vesicles. The
functional units of the gland are called follicles. The thyroid of a normal rat contains about
100,000 follicles. These are separated by interfollicular connective tissue containing
numerous capillaries. Each follicle consists of single peripheral layer of cuboidal cells
surrounding a cavity filled with a colloid.

 Fig1: Thyroid gland Fig 2: Histological features of Thyroid
Vesicles are mainly responsible for the production of thyroid hormones whereas colloids
are made up of a large complex glycoprotein called thyroglobulin. The hormones remain
bound to thyroglobulin until secreted. . When they are secreted, colloid is ingested by the
thyroid cells, the peptide bonds are hydrolyzed and free T3 and T4 are discharged into the
capillaries.
Hormones of thyroid:
Thyroid secretes two hormones:
1. Thyroxine
2. Calcitonin or thyrocalcitonin
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1. ThyroxineThyroid hormone consists of at least four physiologically active iodinated organic
compounds collectively called iodothyronine or thyroid hormone. The most abundant
(93%) and important of the four is called thyroxine or tetraiodothyronine (T4), which until
recently was believed to be the only physiologically active component or hormone secreted
by the gland.
The raw materials for the production of these two hormones are called tyrosin (amino acid)
and iodine. The gland can collect and store iodine until the concentration inside the cell is
at least 25 times than that in the circulating blood.
 Fig 3: Synthesis of Thyroid hormone
Before the iodide can be incorporated, it must be oxidised (I0
) by a peroxidase system of
enzymes, which is under the influence of TSH.
2. Thyrocalcitonin or calcitonin:
In human being thyroid also secrets calcitonin which is a large polypeptide with a
molecular weight 3400 and a chain of 32 amino acids. Calcitonin is synthesized in
parafollicular cells ‘C’ cells present in the connective tissue around thyroid follicular cells.
It is associated with calcium phosphate metabolism. TCT act by causing a transfer of
calcium from blood into bone by increased calcification of the bones or by diminished
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decalcification or by both mechanisms. This function of TCT is opposed by the hyper
calcemic hormone produced by the parathyroid.
Functions of thyroid hormone:
1. Increased metabolic activities- it increase the rate of O2 consumption or
oxidative metabolism and resulting heat production of the various cells and tissues of the
body. It stimulates many phases of carbohydrate metabolism.
2. Regulatory role- Thyroid hormone has important role in growth and
development of cells and tissues. These include the sexual development, maturation of
bones, mental development and energy metabolism. In absence of thyroid hormone,
growth hormone secretion is also depressed and thyroid hormones potentiate the effect of
growth hormone on tissues.
3. Effect on other endocrine glands- Increased thyroid secretion increases the rate
of secretion of most other endocrine glands, but it also increases the needs of the tissues for
hormones.
4. Effect of calcitonin on blood calcium- Calcitonin reduces the blood calcium ion
concentration (an effect, which is opposite to that of parathyroid hormone).
 But calcitonin mechanism acts only weekly and only as a short-term regulator of
calcium iron concentration because it is rapidly overridden by the much more powerful
parathyroid mechanism.
5. Other role- Thyroid hormones are essential for hepatic conversion of carotene to
vitamin A. The accumulation of carotene in the blood stream (carotenemia) in
hypothyroidism is responsible for yellowish tint of the skin.