The document discusses the anatomy, histology, blood supply, innervation, and functions of the thyroid and parathyroid glands. The thyroid gland synthesizes and secretes thyroid hormones including thyroxine (T4) and triiodothyronine (T3). It is located in the neck and has a blood supply from the superior and inferior thyroid arteries. The parathyroid glands regulate calcium levels and are usually located behind the thyroid. Common diseases of the thyroid include hyperthyroidism and hypothyroidism.

1. Thyroid gland
Prepared by
Dr.Ismael I. Surchi

2. Objectives
Anatomy and histology of thyroid gland.
Physiological function of thyroid gland.
Blood supply and Innervation of thyroid gland.
Anatomy of Parathyroid glands with their blood and
nerve supply.
Synthesizes, storage and secretion of thyroid
hormones.
Physiological effects of thyroid hormones.
Disease of thyroid gland.
Other condition of thyroid gland.

3.  The thyroid gland is located
immediately below the larynx
and anterior to the upper part
of the trachea. It weighs
about 25g.
 It consists of 2 lateral lobes
connected by a narrow band
of thyroid tissue called the
isthmus.
 The isthmus usually overlies
the region from the 2nd to 4th
tracheal cartilage.
 The sternocleidomastoid
muscle and the three strap
muscles (sternohyoid,
sternothyroid, and the
superior belly of the
omohyoid) border the thyroid
gland anteriorly and laterally

5. HISTOLOGY
The lobes of the thyroid
contain many hollow,
spherical structure called
follicles, which are the
functional units of the thyroid
gland.
 Interspersed between the
follicles are C cells, which
secrete calcitonin.
Each follicle is filled with a
thick sticky substance called
colloid.

6. Arterial blood supply
 Superior thyroid artery is the first branch
off the external carotid artery. It extends
inferiorly to the superior pole of the
thyroid lobe. In addition to supplying the
thyroid, the superior thyroid artery is the
primary blood supply to approximately 15
percent of superior parathyroid glands.
 Inferior thyroid artery is a branch of the
thyrocervical trunk which arises from the
subclavian artery. The inferior thyroid
artery courses posterior to the carotid
artery to enter the lateral thyroid. The
inferior thyroid artery also supplies the
inferior parathyroid glands and
approximately 85 percent of superior
parathyroid glands.
 Thyroidea ima artery is found in
approximately 3 percent of individuals
and branching directly from subclavian
artery, inferior portion of the isthmus or
inferior thyroid poles.

7. Venous drainage
 Superior thyroid vein:
 It arises from the upper part of the
lobe.
 It ends into the internal jugular
vein.
 Middle thyroid vein:
 It arises from the middle of the
lobe.
 It ends into the internal jugular
vein.
 Inferior thyroid veins:
 Arise from the isthmus and lower
parts of the lobes.
 Descend in front of the trachea.
 End into the left brachiocephalic
vein

8. Lymphatic drainage
 The lymphatic drainage of the thyroid is multidirectional and extensive.
 It drains initially into peri-thyroid nodes, and from there int prelaryngeal,
pretracheal and paratracheal nodes.
 Laterally, the gland drains into the superior and inferior deep cervical nodes.

9. Nerve supply
 Principal innervation of the
thyroid gland derives from the
autonomic nervous system.
 Sympathetic innervation:
 The thyroid gland is provided by
fibers from the superior and
middle cervical sympathetic
ganglia.
 The fibers enter the gland with
the blood vessels and are
vasomotor in action.
 Parasympathetic:
 Fibers are derived from the
vagus nerve and reach the gland
via branches of the laryngeal
nerves.

11. Parathyroid gland

13. Size and location
 Normal parathyroid glands are approximately the size of a
grain of rice or a lentil.
 Normal glands are usually about 5 by 4 by 2 millimeters in
size and weigh 35 to 50 milligrams.
 Enlarged parathyroid glands can be 50 milligrams to 20
grams in weight, most typically weighing about 1 gram and 1
centimeter in size

14. Function of parathyroid gland

15. Superior parathyroid gland
Normal superior parathyroid glands are usually
located on the posterior-lateral surface of the
middle to superior thyroid lobe.
They lie under the thyroid superficial fascia,
posterior to the recurrent laryngeal nerve and can
be visualized by carefully dissecting the thyroid
capsule in this region.
The superior parathyroid glands receive most of
their blood supply from the inferior thyroid artery
and also are supplied by branches of the superior
thyroid artery in 15 to 20 percent of patients.

16. Inferior parathyroid gland
The two inferior parathyroid glands reside in the
anterior mediastinal compartment, anterior to the
recurrent laryngeal nerve.
They are most often found in the thyrothymic tract,
or just inside the thyroid capsule on the inferior
portion of the thyroid lobes
The inferior parathyroid glands receive their end-
arterial blood supply from the inferior thyroid artery.

17. The thyroid gland synthesizes
and secretes three hormones:
• Thyroxine (T4).
• Tri-iodothyronine (T3).
• Calcitonin

18. Thyroid Physiology
Synthesis of the thyroid hormone
 Tyrosine and iodine are essential for synthesis of thyroid hormones.
 Both are taken up by the blood.
 Tyrosine is synthesised by the body (in the thyroglobulin).
 Iodine is a dietary essential.
 Binding of iodine with tyrosine to form iodotyrosines.
 Hormone synthesis occurs on the thyroglobulin.

20.  Thyrotropin-releasing hormone (TRH) increases the
secretion of thyrotropin (TSH), which stimulates the synthesis
and secretion of trioiodothyronine (T3) and thyroxine (T4) by
the thyroid gland.
 T3 and T4 inhibit the secretion of TSH, both directly and
indirectly by suppressing the release of TRH.
 T4 is converted to T3 in the liver and many other tissues by
the action of T4 monodeiodinases.
 Some T4 and T3 is conjugated with glucuronide and sulfate
in the liver, excreted in the bile, and partially hydrolyzed in
the intestine.
 Some T4 and T3 formed in the intestine may be reabsorbed.

21. Following steps are involved synthesis, storage
and secretion:
1. Thyroglobulin production by follicular cell and released into
colloid by exocytosis
2. Iodine uptake by follicular cell from the blood and transferred to
colloid
3. Attachments of iodine to tyrosine on thyroglobulin in colloid
4. Coupling processes between the iodinated tyrosine molecules to
form T4 and T3
5. Secretion (upon stimulation) of T4 and T3 occurs by endocytosis
a piece of colloid, uncoupling of T4 and T3 and diffusion out of the
follicular cell into the blood

22.  Approximately, 90% of the hormones released from the
thyroid gland initially appear in the form of T4.
 However, a majority of the T4 that is secreted from the
thyroid gland is subsequently converted to T3.
 T3 is 4times more potent in its biologic form than T4
and is the major hormone that interacts with the target
cells.
 The metabolic effects of the thyroid hormones are due
to unbound T4 and T3.
 T3 is quick acting (within a few hours) whereasT4 acts
more slowly (4-14 days)

23. Both the hormones are highly lipophilic and once
in the blood, immediately bind to proteins:
 - Thyroid hormone specific protein- thyroxine
binding globulin (70-80%)
 - Other non-specific proteins
 - Less than 0.1% of T4 and 1% of T3 are in
unbound form.
- These free T4 and T3 compounds are
biologically active, and it is these components that
produce the effects of the thyroid hormones on
peripheral tissues and on the pituitary feedback
mechanism.

25. PHYSIOLOGICAL EFFECTS OF THYROID
HORMONES

26. DISEASES OF THE THYROID
GLAND
Thyroid hormones can be either:
deficient or excessive
Hyperthyroidism.
Hypothyroidism

27. Hyperthyroidism
 Excessive thyroid hormone
production. Hyperthyroidism is
most often caused by Graves’
disease or an overactive thyroid
nodule.

28. Hypothyroidism
 Low production of thyroid
hormone. Thyroid damage
caused by autoimmune disease
which is the most common cause
of hypothyroidism .

29. Other Thyroid Conditions
 Goiter : A general term for thyroid swelling. Goiters can be
harmless, or can represent iodine deficiency or a condition
associated with thyroid inflammation called Hashimoto’s thyroiditis.
 Thyroiditis : Inflammation of the thyroid, usually from a viral
infection or autoimmune condition. Thyroiditis can be painful, or
have no symptoms at all.
 Graves disease : An autoimmune condition in which the thyroid is
overstimulated, causing hyperthyroidism.
 Thyroid cancer : An uncommon form of cancer, thyroid cancer is
usually curable. Surgery, radiation, and hormone treatments may
be used to treat thyroid cancer.
 Thyroid nodule : A small abnormal mass or lump in the thyroid
gland. Thyroid nodules are extremely common. Few are cancerous.
They may secrete excess hormones, causing hyperthyroidism, or
cause no problems.
 Thyroid storm : A rare form of hyperthyroidism in which extremely
high thyroid hormone levels cause severe illness.

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