2. CONTENT
Anatomy of the thyroid gland
Synthesis and secretion of thyroid hormones
Regulation of thyroid hormone secretion
Physiological functions of the thyroid hormone
Diseases of the thyroid
3. The endocrine system consists of ductless glands
that produces hormones which pass into the blood
stream for general circulation to excite or inhibit the
activity of other organs or tissues.
The thyroid gland is one of the largest endocrine
glands.
It secretes 2 major hormones:
Thyroxine – T4
Triiodothyronine – T3
4. ANATOMY
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.
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. The major constituent of colloid is a large
glycoprotein called thyroglobulin.
Unlike other endocrine glands, which secretes their
hormones once they are produced, the thyroid gland
stores considerable amount of the thyroid hormones
in the colloid until they are needed by the body.
7. SYNTHESIS, STORAGE & SECRETION OF
THYROID HORMONES
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.
Hormone synthesis occurs on the thyroglobulin.
8. SYNTHESIS, STORAGE & SECRETION
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, uncouplingof T4 and T3 and diffusion out of the
follicular cell into the blood
10. SYNTHESIS, STORAGE & SECRETION cont..
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.
11. SYNTHESIS, STORAGE & SECRETION cont..
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.
14. PHYSIOLOGICAL EFFECTS OF THYROID
HORMONES
Metabolic rate and heat production:
↑ metabolic activities
↑ O₂ consumption to most metabolically active tissues
BMR can ↑ by 60 – 100%
Since ↑ metabolism results in ↑ heat production → thyroid
hormone effects is calorigenic
Intermediary metabolism:
Modulates rates of many specific reactions involved in fuel
metabolism
15. PHYSIOLOGICAL EFFECTS OF THYROID
HORMONES
Sympathomimetic effect
Sympathomimetic: any action similar to one produced by the
sympathetic nervous system
Thyroid hormone ↑ target cell responsiveness to catecholamines
(SNS and adrenal)
Thyroid hormones are permissive → ↑ production of specific
catecholamine target cell receptors
The cardiovascular system:
↑ the heart’s responsiveness to circulating catecholamines
(permissive action above)
↑ heart rate and force of contraction → ↑ CO
In response to the heat load → peripheral vasodilation to eliminate
generation of extra heat
16. PHYSIOLOGICAL EFFECTS OF THYROID
HORMONES
Growth and the nervous system
Effect on growth is manifested mainly in growing
children
Thyroid hormone stimulates GH secretion and promotes GH effects
Lack of thyroid hormone → growth retardation but can be reversed
by thyroid replacement therapy
Unlike excess GH, excess thyroid hormone does not produce
excessive growth
Important in promoting growth and development of the
brain during fetal and postnatal life
Thyroid hormone deficiency → mental retardation if therapy is not
administered days or weeks after birth
18. DISEASES OF THE THYROID GLAND
Thyroid hormones can be either deficient or excessive
– hypothyroidism or hyperthyroidism.
19. UNDERACTIVITY OF THYROID
If the thyroid show atrophy or destruction of its
secretory cells or is inadequately stimulated, the
syndrome of hypothyroidism develops because of
lack of thyrotrophin releasing hormone from the
hypothalamus or TSH from the anterior pituitary
gland.
20. HYPOTHYROIDISM
Symptoms:
↓ overall metabolic activity, including ↓ BMR
Poor tolerance of cold → lack of calorigenic effect
Tendency to gain weight → not burning fuel at a
normal rate
Gut movements sluggish →constipation
Easily fatigue → lower energy production
Heart and respiratory rates and BP reduced.
Mental effects including ↓ alertness, slow speech
and poor memory.
21. HYPOTHYROIDISM
Symptoms:
Hair- brittle, sparse, dry.
Slow, husky voice.
Myxedema → Retention of water presumably by
water retaining carbohydrates as a result of altered
metabolism.
Generically, treatment consist of replacement therapy:
Thyroid hormone
Iodine
23. HYPERTHYROIDISM
Most common cause –
Grave’s disease
Autoimmune disease where
thyroid-stimulating
immunoglobulin (TSI)
is produced
erroneously
Is not subject to the
negative feedback
mechanism
24. HYPERTHYROIDISM
Symptoms:
BMR is ↑, more heat is produced → ↑body
temperature
Skin is hot and flushed
Appetite increase but weight falls
Movements of digestive tracts are ↑ → diarrhea
Heart and respiratory rates ↑
↑ BP
A fine muscular tremor and nervousness are marked.
Person becomes excitable, irritable and apprehensive.
25. Symptoms:
Exophthalmos →
complex water-retaining
carbohydrate are deposited
behind the eye → fluid
retention pushes eyeballs
forward (prominent feature
of Grave’s disease only).
Treatment for
hyperthyroidism is anti-
thyroid drugs.
26. THYROID DISEASE
Goiter
May occur in
hypothyroidism and
hyperthyroidism but does
not need to be present in
either conditions
27. CASE STUDY
A 39-year-old woman presented with a 3month
history of increased sweating and palpitations with
weight loss of 7kg. She had family history of thyroid
disease, her maternal aunt had suffered from thyroid
disease.
On examination, she was a nervous, agitated woman
with an obvious, diffuse, non-tender, smooth
enlargement of her thyroid, over which a bruit could
be heard. She had a fine tremor of her fingers and a
resting pulse rate of 150/minute. She had no
evidence of exophthalmos.
28. CASE STUDY cont…
On investigation, she had a raised serum T3 of
4.8nmol/l (NR 0.8-2.4) and a T4 of 48nmol/l (NR 9-
23). Measurement of her thyroid-stimulating
hormone showed that this was low normal, 0.4mU/l
(NR 0.4-5_mU/l). The biochemical findings pointed
to primary thyroid disease.
A diagnosis of autoimmune thyrotoxicosis (Graves'
disease) was made. She was treated with an
antithyroid drug, carbimazole, to control her
thyrotoxicosis, and surgery was not required.
29. REFERENCES:
Callander R, Mackenna B: Illustrated Physiology,
6th Edition
Ganong, WF: ‘Review of Medical Physiology’, 23rd
Edition
Guyton, AC and Hall JE: ‘Textbook of Medical
Physiology’, 12th Edition
Tortora, GJ and Derrickson, B: ‘Principles of
anatomy and physiology, 13th Edition