1.
BY: SAFANA SADIQ
PHYSIOLOGY OF THE
THYROID GLAND

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

9.
SYNTHESIS, STORAGE & SECRETION cont..

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.

12.
REGULATION OF THRYOID HORMONE
SECRETION

13.
REGULATION OF THYROID HORMONE
SECRETION

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

17.
PHYSIOLOGICAL EFFECTS OF THYROID
HORMONES

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

22.
HYPOTHYROIDISM

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

30.
THE END