Advanced in thyroid gland.

1. THE THYROID GLAND
KRISHNPRADDHUMN
(B.Sc. Sem5th)
LucknowChristianCollege,Lucknow

2. Introduction :
The thyroid gland is a butterfly-shaped organ located in the base of
your neck, below the Adam's apple (anterio-ventral region). It releases
hormones that control metabolism—the way your body uses energy.
Microscopically, the functional unit of the thyroid gland is the
spherical thyroid follicle.
The thyroid gland develops in the floor of the pharynx at the base of
the tongue at 3–4 weeks gestation; it then descends in front of the
pharyngeal gut, and ultimately over the next few weeks, it migrates to
the base of the neck.
During migration, the thyroid remains connected to the tongue by a
narrow canal, the thyroglossal duct.
 At the end of the fifth week the thyroglossal duct degenerates, and
over the following two weeks the detached thyroid migrates to its final
position.

3. HISTORY
 Galen firstly described some
secreting glands adjacent to the thyroid cartilage.
 Eustachius named these glands as laryngeal gland.
 Wharton in 1656 named them glandulae
thyroidaeae (Thyroid glands) because of their
anatomic proximity to thyroid cartilage.
 During 1700s it was discovered that goiter was a
disease of the thyroid.
Ref. web: 1.1

4. Development
 The thyroid gland is the first endocrine gland
to develop in the embryo.
 It begins to form about 24 days after
fertilization.
 It appears as a median endodermal thickening
in the floor of the primordial pharynx between
tuberculum impar and copula.
 The site from which it originated persists as
the foramen cecum at the base of the tongue.
Ref. web: 1.2

5. Fig. 1: to show the development of thyroid gland
Ref. Web: 2.0

6. ANATOMY
 The thyroid gland is highly vascularized organ located
anteriorly in the neck between the C5 and T1 vertebrae, deep
in the platysma, sternothyroid, and sternohyoid muscles.
 The thyroid weighs 15–20 g and weighs more in men than in
women; the thyroid weighs approximately 1 g in a newborn and
increases by about 1 g/year until age 15.
 It is an H-shaped, soft, and reddish parenchymal organ,
consisting of two lobes (left and right) and one isthmus that
binds them together (Fig. 1).
 Each lobe is approximately 4 cm in length, 2 cm in width, and
2–3 cm in thickness.
 The isthmus measures about 2 cm in width, 2 cm in height, and
2–6 mm in thickness
Ref.: E-BOOK 1.1

7. CONT…
 The superiorly (called the superior horn) lies lateral
to the inferior constrictor muscle and posterior to the
sternothyroid muscle, while the inferior part (inferior
horn) extends to the levels of the fifth or sixth
tracheal ring.
 In the posterolateral section, the gland overlaps the
carotid sheath and its components.
 About 50% of individuals present a pyramidal lobe
(Morgagni’s or Lalouette’s pyramid), arising from
either lobe or the superior portion of the isthmus and
directed upward, usually to the left.

8. Cont…
 The thyroid is enveloped by the layers of the deep cervical fascia and
covered by the strap muscles anteriorly and the sternocleidomastoid
muscle more laterally.
 The true thyroid capsule is firmly adherent to the gland, developing
projections into the thyroid, forming septae and dividing it into lobes
and lobules.
 The posterior layer of the thyroid capsule is thick.
 Posteriorly, the middle layer of the deep cervical fascia condenses to
form the posterior suspensory ligament of Berry, connecting the thyroid
lobes to the cricoid cartilage and the first two tracheal rings.
 In the posterior surface of the lateral lobes are located the parathyroid
glands; normally there are four (two superior and two inferior), and
these are roundish, and about the size of a grain of rice

9. Copyright -1.1
Fig.2: THE THYROID GLAND
Image Ref.: E-BOOK 1.1

10.  Histologically this gland consists follicles, are
held together with the help of connective
tissue.
 Cells of follicle are single cuboidal, epithelial,
cavity of follicle is filled by yellowish jelly like,
homogenous or heterogenous colloidal
material (Iodinated colloid Glycoprotein).
 This gland contains blood vessels and
parafollicular cells.
Histology of ThyroidGland
Ref.: E-BOOK 1.1

11. Secretion
 The Thyroid Hormone is stored as colloid in the
inactive form.
 When required, an enzyme (Thyroid peroxydases)
activates thyroglobulin which breaks into its
components.
 Thyroxine T4 and Triiodothyronine T3 when released
in blood are at once tied up with blood proteins. Thus
Thyroid hormones travel in blood as Protein-bound-
iodine (PBI) 4-8mg/100 ml plasma.
 Upon reaching the target tissue PBI is broken down
to set the hormones free where T4 changes to T3 .

12. Fig.3: Histology of thyroid gland

13. Electron Microscopy of Thyroid follicles
 The thyroid gland have many thyroid
follicles of variable size.
 Each follicle was lined by a single layer of
cuboidal follicular cells with round vesicular
nuclei and surrounded a lumen filled with
acidophilic colloid.
 Groups of interfollicular cells were
encountered in the interfollicular spaces.
Ref. Web: 1.4

14. Fig.4: electron microscopy of
thyroid follicle
(a) (b)

15. Hormones of Thyroid gland
 The Thyroid gland is related with
secretion of 2 iodinated hormones and a
non iodinated hormone:
A. Triiodothyronine (T3).
B. Tetraiodothyronine (T4) or Thyroxine.
C. Thyrocalcitonin (Noniodinated
hormone)

16. Iodides and Iodines
 An adult human being contains about 5-6
mg of iodine, mostly it is stored in Thyroid
gland (Reservoir of Iodine).
 Thyroid utilizes about 150 mg of Iodine.
 A person must daily obtain 150 micrograms
of Iodine from the food.
 Iodine of food is absorbed and circulated in
the blood in the form of Iodide ion(I ).
-

17. SYNTHESIS OF THYROXINE
 Thyroxine Hormone derived from a
glycoprotein (Thyroglobulin).
 Each molecule of Thyroglobulin contains
about 500 AA monomers of which 123
monomers are Tyrosine at fixed places.
 There is incorporation of Iodine with
Tyrosine as given below:

18. Tetraiodothyroglobulin or triiodothyronine =
Thyroxine

19. Fig.5: .Synthesis of thyroid hormones. Iodine is actively concentrated by thyroid cells. An enzyme called thyroperoxidase catalyses the
addition of iodine to the tyrosine residues in thyroglobulin, a large protein, rich in tyrosine residues, which is synthesized in thyroid cells. The
iodinated thyroglobulin is stored in the thyroid in the form of ‘colloid’. In response to TSH stimulation, portions of the colloid are taken back into
the thyroid cell by phagocytosis and pairs of iodinated tyrosine residues (thyroxine) are released into the circulation. Anti-thyroid drugs, such as
carbimazole, act by inhibiting thyroperoxidase activity. AC, adenylyl cyclase; cAMP, cyclic adenosine monophosphate.
See reference (1.1)

20. Affects of Thyroid (on metabolism)
 In young animals, Thyroid secretion stimulates tissue
differentiation and growth.
 Frog's tadpole in which thyroid has been destroyed,
fail to metamorphose.
 At the same time, metamorphosis can be has turned
by adding few drops of thyroid extract or iodine in
water.
 Activity of thyroid, below normal (Hypothyroidism),
either due to inactivity of thyroid gland or deficiency
of iodine in in the diet, retards growth in young
animal the teeth are slow to form and erupt.

21. Cont…
 In the adult it causes a lower rate of metabolism, thickening of
the skin and subcutaneous tissue, sluggish growth, low-
temperature, intellectual deficiency, protruded abdomen, and
retarded sexual development.
 Marked symptoms, just described, cause a condition known as
cretinism, when they occur during development (upto 15 years
of age), and myxoedema (gull's disease) in adult.
 Under-secretion leads usually not to the shrinkage, but
enlargement of thyroid tissue itself, which protrudes as mug
swelling below neck, popularly known as simple or endemic
goitre.
 Other symptoms of ,Myxoedema are falling of hair and teeth,
cracking of skin and form of albinism called leucoderma and
vitligo.
Ref. Book: 1.2

22. Cont…
 An over secretion thyroid hyperthyroidism due to
overactive or enlargement gland, producers rivers
symptoms such as high metabolic rate, fast pulse, warm
skin and raised nervous excitability.
 Due to high rate of metabolism, fats are deposited in the
orbits leading to protruded eyes, a condition referred to as
exophthalamos (Grave's or Basedow's disease).
 Such symptoms either occur because of hereditary
enlargement of the gland or from the development of
tumor of thyroid cells.
 Both over- and under activity e of thyroid gland can
produce neck goitre.

23. Cont…
 In special form of hypothyroidism called
hashimoto's disease, antibodies develop
against own thyroglobulin.
 The disease is inherited as a Mendelian
dominant characterstic. It is an instance
of auto-immune disease.

24. THYROGLOSSAL CYST
 A thyroglossal cyst is a fibrous cyst that forms from a
persistent thyroglossal duct.
 Thyroglossal cysts can be defined as an irregular
neck mass or a lump which develops from cells and
tissues left over after the formation of the thyroid gland
during developmental stages.
 Thyroglossal cysts are the most common cause of
midline neck masses and are generally
located caudal to (below) the hyoid bone.
 Thyroglossal cysts develop at birth.

25. Fig. 1 : image to show Thyroglossal cyst
Ref.: Image web: 1.3

26. Conclution
 Thyroid gland is important for body metabolism:
Thyroid gland is directly associated with the
metabolism.
 Thyroid hormones may considered as tempo of life:
It secretes hormone thyroxine, which consists Iodine,
which is associated with metamorphosis in lower, as
well as higher vertebrates.
 Frog’s tadpoles in which thyroid has been destroyed,
fails to metamorphosis.
 In no other endocrine glands than thyroid gland
hormones are stored.

27. Reference
 BOOK 1.1: The Endocrine System
PUBLISHERS: ELSEVIER
AUTHORS:
Joy Hinson (BSc PhD DSc FHEA)
Peter Raven (BSc PhD MBBS MRCP MRCPsych FHEA)
Shern Chew (BSc MD FRCP)
BOOK 1.2: Biology Of Chordates
Publishers: VISHAL
Author: Harish C. Nigam (M.Sc.(Lko.), Ph.D (Herpetol),FIAZ)
WEB 1.1: https://www.wikipedia.org
Web1.2: https://www.ncbi.nlm.nih.gov/books/NBK551611/
Web 1.3:https://www.image.google.com/
Web 1.4: https://researchgate.net Web 2.0: https://www.springer.com/in