Thyroid Gland (Anatomy, Hormones, Disorders).pdf

*Thyroid Gland
*Anatomy of Thyroid Gland
*Regulation of Thyroid Hormones Secretion
*T3 and T4
*Calcitonin
*Hypothyroidism
*Hyperthyroidism

1. Thyroid Gland
•The thyroid is a butterfly-shaped gland. It is
positioned on the neck just below the Larynx
and has two lobes, one on either side of the
trachea. The two lobes are connected by a
bridge (isthmus) in the middle.
•Brownish-red in color, and rich with blood
vessels.
•It is involved in the production of the following
hormones:
1)Tetraiodothyronine (T4 or thyroxine)
2)T3 (triiodothyronine)
3)Calcitonin

3. Anatomy
•The lobes of the gland, as well as the isthmus,
contain many small globular sacs called follicles.
The follicles are lined with follicular cells and are
filled with a fluid known as colloid that contains
stored thyroglobulin (Tg), a glycoprotein. When
thyroid hormones are needed, thyroglobulin is
reabsorbed from the colloid into the follicular cells,
where it is used to produce the two thyroid
hormones thyroxine (T
4) and triiodothyronine (T
3).
The hormones are then released, passing from the
cells into the circulation.
•The Para-follicular cells, also known as C-cells,
secrete the hormone Calcitonin.

4. Regulation of Thyroid Hormones
Secretion
•The release of T3 and T4 from the thyroid gland is
regulated by thyroid stimulating hormone (TSH).
•Low blood levels of T3 and T4 stimulates the release
of thyrotropin-releasing hormone (TRH) from the
hypothalamus, which triggers secretion of TSH from
the anterior pituitary.
•In turn, TSH stimulates the thyroid gland to secrete T3
and T4.
•The levels of TSH, T3, and t4 are regulated by
negative feedback system in which increasing levels
of T3 and T4 decrease the production and secretion
of TSH.

6. T3 and T4
•These are iodinated amino-acid hormones. Iodine
combines with the amino acid tyrosine to make
these hormones.
•These are lipid-soluble hormones.
•T3 and T4 are responsible to maintain metabolism
throughout the body, and that’s why they are often
referred to as metabolic hormones.
•They are also crucial in human growth and
development.
•They are especially critical for normal development
of the nervous system both in utero and in early
childhood, and they continue to support
neurological function in adults.

7. •Thyroid hormones affect every cell and all the
organs of the body. They:
•Regulate the rate at which calories are burned,
affecting weight loss or weight gain.
•Can slow down or speed up the heartbeat.
•Can raise or lower body temperature.
•Influence the rate at which food moves
through the digestive tract.
•Control the way muscles contract.

8. Calcitonin
•Calcitonin is also called thyrocalcitonin, a hormone that
is produced and released by the C-cells or Para-follicular
cells of the thyroid gland.
•It is a peptide hormone, and water soluble.
•Calcitonin plays a role in regulating calcium levels in the
blood, which is important for your bone health and
maintenance.
•It acts to reduce calcium levels in the blood. However,
the importance of this role in humans is unclear, as
patients who have very low or very high levels of
calcitonin show no adverse effects.
•Secretion of this hormone is controlled directly by the
blood’s calcium levels. When the levels start to increase,
the body responds with increased calcitonin levels.
When calcium levels drop, so do calcitonin levels.

9. Calcitonin reduces calcium levels in the
blood by two main mechanisms:
•It inhibits the activity of osteoclasts, which are
the cells responsible for breaking down bone.
•When bone is broken down, the calcium
contained in the bone is released into the
bloodstream. Therefore, the inhibition of the
osteoclasts by calcitonin directly reduces the
amount of calcium released into the blood.
•It can also decrease the reabsorption of calcium
in the kidneys, again leading to lower blood
calcium levels, resulting in increased calcium
excretion in the urine.

10. Hypothyroidism
•Hypothyroidism (underactive thyroid) - not enough
thyroxine is produced for the body’s needs.
•Hypothyroidism can result from:
1.Primary failure of thyroid gland.
2.Deficiency of TRH, TSH or both.
3.Inadequate dietary supply of iodine.
•If too little of the thyroid hormones are produced,
the cells and organs of your body slow down. The
metabolic activity of the body slows down, which
causes weight gain (not burning fuel at a normal
rate), easily fatigued (low energy production), poor
tolerance to cold, slow mental responsiveness, and
poor memory.

11. Hyperthyroidism
•Hyperthyroidism (overactive thyroid) - too much
thyroxine is produced for the body’s needs.
•It is often caused by a pituitary or thyroid tumor.
•If you become hyperthyroid because of too
much secretion of the hormones from the
thyroid gland, there is an increase in the activity
of your body cells or body organs.
•This condition can make your metabolism speed
up. Symptoms of hyperthyroidism include a rapid
heartbeat, weight loss, increased appetite and
anxiety.