Anatomy -Thyroid gland

1. Anatomy-THYROID GLAND

3. THE 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.

4. It is a highly vascular gland (blood
supply).
that weighs about 25 g
and is surrounded by a fibrous capsule.

5. It resembles a butterfly in shape,
consisting of two lobes, one on either
side of the thyroid cartilage
and upper cartilaginous rings of the
trachea.

6. The lobes are joined by a narrow
isthmus, lying in front of the trachea.

7. The lobes are roughly cone-shaped,
about 5 cm long
and 3 cm wide.

8. Arterial blood supply
The arterial blood supply to the gland is
through
--the superior
and inferior thyroid arteries.

9. The superior thyroid artery is a branch of
the external carotid artery
and the inferior thyroid artery is a branch
of the subclavian artery.

10. The venous return
The venous return is by the thyroid
veins which drain into the internal
jugular veins.

11. Two parathyroid glands lie against the
posterior surface of each lobe
and are sometimes embedded in thyroid
tissue.

12. The recurrent laryngeal nerve passes
upwards close to the lobes of the gland
and on the right side it lies near the inferior
thyroid artery.

13. The gland is composed of cuboidal
epithelium that forms spherical follicles.

14. These secrete and store colloid,
a thick sticky protein material.

15. Between the follicles there are other cells
found singly or in small groups:
parafollicular cells, also called C-cells,
which secrete the hormone calcitonin.

16. THYROXINE AND
TRIIODOTHYRONINE

17. Iodine
Iodine is essential for the formation of the
thyroid gland hormones,
Thyroxine (T4) and
Triiodothyronine (T3).

18. Main sources of iodine
The body's main sources of iodine are
seafood,
vegetables grown in iodine-rich soil
and iodinated table salt in the diet.

19. iodine trapping
The thyroid gland selectively takes up iodine
from the blood,
a process called iodine trapping.

20. Thyroglobulin
The thyroid hormones are synthesised as
large precursor molecules called
thyroglobulin,
the major constituent of colloid.

21. The regulation of T3 and T4
The release of T3 and T4 into the blood is
regulated by thyroid stimulating hormone
(TSH) from the anterior pituitary.

22. Secretion of TSH
Secretion of TSH is stimulated by the
thyroid releasing hormone (TRH) from the
hypothalamus.

23. and secretion of TRH is stimulated by
Exercise,
Stress,
Malnutrition,
Low plasma glucose
And sleep.

24. The level of secretion of TSH depends on
the plasma levels of T3 and T4 because
these hormones affect the sensitivity of
the anterior pituitary to TRH.

25. Increased levels of T3 and T4 decrease TSH
secretion and vice versa.

26. When the supply of iodine is deficient,
excess TSH is secreted and there is
proliferation of thyroid gland cells and
enlargement of the gland.

27. Secretion of T3 and T4 begins about the
third month of fetal life and is increased at
puberty and in women during the
reproductive years, especially during
pregnancy.

28. Otherwise, it remains fairly constant
throughout life.

29. Thyroid hormones enter the target cells and
regulate the expression of genes in the
nucleus,

30. i.e. they increase or decrease the synthesis
of some proteins including enzymes.

31. They combine with specific receptor sites
and enhance the effects of other
hormones,
e.g. adrenaline and noradrenaline.

32. T3 and T4 affect most cells of the body by:
Increasing the basal metabolic rate
and heat production.

33. Regulating metabolism of
carbohydrates, proteins and fats.

34. T3 and T4 are essential for normal growth
and development,
especially of the skeleton and nervous
system.

35. Most other organs and systems are also
influenced by thyroid hormones —

36. physiological effects of T3 and T4 on the
heart, skeletal muscles, skin, digestive and
reproductive systems are more evident when
there is underactivity or overactivity of the
thyroid gland.

37. CALCITONIN
This hormone is secreted by the
parafollicular or C-cells in the thyroid gland.

38. It acts on bone and the kidneys to reduce
the --
blood calcium (Ca2+) level when it is raised.

39. It reduces the reabsorption of calcium from
bones and
inhibits reabsorption of calcium by the renal
tubules.

40. Its effect is opposite to that of parathyroid
hormone (PTH, parathormone),
the hormone secreted by the parathyroid
glands.

41. Release of calcitonin is stimulated by an
increase in the blood calcium level.

42. This hormone is important during childhood
when bones undergo considerable changes
in size and shape.

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