Thyroid gland is such an organ which is present in both mammalian and non-mammalian species. Although it’s a single organ, the thyroid gland of most species has distinct anatomy. Here only human thyroid will be discussed.

1. THYROID GLAND
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2. INTRODUCTION
§ Thyroid gland is such an organ
which is present in both
mammalian and non-mammalian
species. Although it’s a single
organ, the thyroid gland of most
species has distinct anatomy.
§ Here only human thyroid will be
discussed.

3. DEVELOPMENT
Thyroid gland is an embryonic
derivative of the alimentary tract. A
median, unpaired sac live
evagination first appeared from the
floor of the embryonic pharynx.
The distal end of the outgrowth
gradually expands and becomes
blobbed & the stalked attachment
narrows to form the thyroglossal
duct.
The duct usually atrophies & two
lobes become a solid mass of
tissue. Remain connected to each
other by a narrow isthmus of
tissue.

4. POSITION
• Thyroid gland is located
beneath larynx and over
the 2nd, 3rd, 4th & 5th
(partially) tracheal ring.

5. ANATOMY
Thyroid gland is the largest
endocrine gland in human
body.
Butterfly shaped thyroid is
consists of two lobes (left &
right).
Both thyroid lobes are
connected by isthmus.

7. Thyroid follicles (TF) are
the predominant
structures within the
thyroid gland.
Thyroid follicles are
normally filled with a
homogeneous,
eosinophilic, protein-rich
fluid called colloid.

8.  Thyroid follicles are lined by a
single layer of cuboidal thyroid
follicular epithelium (arrows).
 The central lumina are filled with
colloid (eosinophilic/pink material).
 Thyroid follicular epithelial cells
produce and reabsorb products in
colloid, which can be seen as the
non-staining vacuoles, called
resorption lacunae, that are
associated with the luminal
membrane of the epithelial cells
(asterisk).

9. C cells (also known
as thyroid medullary
cells or parafollicular
cells) form small
clusters interspersed
between thyroid
follicles.

10. Thyroid gland secrets
following hormones.
• Thyrocalcitonin (also
known as calcitonin)
secreted by parafollicular
cells.
• T3 & T4: Bothe are
known as thyroid
hormone and secreted
by follicular cells. T4 is
tetraiodothyronine or
thyroxine and T3 is
triiodothyronine.

11. • Inhibits the activity of
osteoclasts.
• Stimulates urinary
excretion of Ca2+
• NB: Thyrocalcitonin is
believed to have negligible
importance in normal
human physiology as
parathyroid hormone and
vitamin D are necessory
for Ca2+ homeostasis.

12. Helps in growth and development
of nervous system.
Helps in linear growth and
maturation of bones.
Increases cardiac output.
Increase neoglucogenesis and
glycogenolysis.
 Increases lipolysis.
Increase protein catabolism in
skeletal muscles.
Increase erythropoietin synthesis.
Increase central stimulation of
respiration.
Increase BMR and oxygen
consumption.

13. HORMONAL REGULATION
OF
THYROID ACTION
The thyroid
hormones T3 and T4 have a number
of metabolic, cardiovascular and
developmental effects on the
body. The production is
stimulated by release of thyroid
stimulating hormone (TSH), which
in turn depends on release of
thyrotropin releasing hormone
(TRH). Every downstream hormone
has negative feedback and
decreases the level of the
hormone that stimulates its
release.

14. THYROID HORMONES
T3 or Triiodothyronine T4 or Tetraiodothyronine

15. T3 & T4 PRODUCTION
Tyrosine (amino acid) is the precursor amino acid for the production of T3 and T4

16. T3 & T4 PRODUCTION
Monoiodotyrosine (MIT) Diiodotyrosine (DIT)
 Tyrosine reacts with oxidized iodide or iodine to produce MIT or DIT.
If one tyrosine molecule reacts with a single iodine atom then MIT forms.
If one tyrosine molecule reacts with two iodine atoms then DIT forms.

17. T3 & T4 PRODUCTION
 MIT and DIT reacts to from T3

18. T3 & T4 PRODUCTION
 Two DIT reacts to form T4

19. T3 & T4 PRODUCTION
 Reaction of thyroid hormone producation
does not occurs at molicular level as
discussed earlier.
 Tyrosine molecule polymerize to form
thyroglobulin.

20. T3 & T4 PRODUCTION
polymerization

21. T3 & T4 PRODUCTION
Representation of thyroglobulin

22. T3 & T4 PRODUCTION
THYROGLOBULIN IODINISED THYROGLOBULIN
CONVERSION OCCURRS AT COLLOID

23. T3 & T4 PRODUCTION

24. T3 & T4 PRODUCTION

25. IODOTHYRONINES FOUND IN HUMAN BLOOD
PLASMA

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