This document provides an overview of the thyroid gland and thyroid hormones. It discusses the functional anatomy and gross anatomy of the thyroid, as well as the histological structure. It explains the biosynthesis and storage of thyroid hormones, their secretion, transport, and metabolism. It covers the regulation of thyroid hormone secretion and applied aspects like hyperthyroidism and hypothyroidism. Key points covered include the roles and actions of the thyroid hormones T3 and T4, iodine metabolism in the thyroid, and thyroid function tests.

1. DR NILESH KATE
MBBS,MD
ASSOCIATE PROF
ESIC MEDICAL COLLEGE, GULBARGA.
DEPT. OF PHYSIOLOGY
THYROID
HORMONE.

2. OBJECTIVES.
 Functional Anatomy.
 Gross anatomy
 Histological structure
 Thyroid hormones
 Biosynthesis & Storage
 Secretion, transport & Metabolism.
 Regulation of thyroid hormone secretion.
 Applied aspects
 Hyperthyroidism
 Hypothyroidism.

3. FUNCTIONAL ANATOMY.
GROSS ANATOMY
 Largest endocrine
gland
 15-25 gm
 Consists of 2 lobes
joined by narrow
Isthmus.
 High blood supply –
400-600ml/100g/min.
Tuesday, February 20, 2018

4. HISTOLOGICAL STRUCTURE
 Each lobe divide3d
into lobules by fibrous
septa.
 Each lobules made up
of several follicles.
 Each follicle lined by
Follicular cells.
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5. HISTOLOGICAL STRUCTURE
 Follicular cells.
 Cuboidal & contains colloid.
 Secrete Thyroid Hormones.
 Parafollicular cells (C cells).
 Between follicular cells
 Secrete Calcitonin.
 Colloid.
 Homogenous material in each
follicle
 Contains Thyroglobulin.
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6. THYROID HORMONES
 2 Principal Hormones
 Tri-iodothyronine T3
 Thyroxine T4
 T3 – 10% , Responsible for
most of tissue actions.
 T4 – 90%
 Calcitonin – hormone
secreted by Parafollicular
cells, concerned with Calcium
Homeostasis.
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7. IODINE METABOLISM
 Dietary Intake – rich sources –
sea fish, bread milk, vegetables
 Avg intake – 500 μg
 Daily Requirement – 100-200
μg.
 Fate – 80% of iodides absorbed
from GIT removed by thyroid
gland.
 Plasma iodide – 0.15 – 0.3 μg.
 Thyroid Iodide – contains 5-8 mg.
(95% of total iodide content)
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8. THYROID HORMONES
BIOSYNTHESIS & STORAGE
 Iodine trapping
 Synthesis &
secretion of
thyroglobulin
 Oxidation of iodides.
 Organification of
thyroglobulins.
 Coupling reaction.
 Storage.
Tuesday, February 20, 2018

9. IODINE TRAPPING
 Uptake of iodide by thyroid
gland.
 By Iodine pump or Na/I –
symporter.
 Thyroid stimulating
hormone – controls iodide
uptake.
 Anti-thyroid agents –
Thiocynate & Perchlorate
inhibit Iodide transport.
Tuesday, February 20, 2018

10. SYNTHESIS & SECRETION OF
THYROGLOBULIN
 Thyroglobulin – large Glycoprotein synthesized in
rough endoplasmic reticulum of thyroid epithelial
cells & then released into lumen.
 1 molecule of Thyroglobulin contains – 140
thyrosine residue which is a substrate for iodine for
thyroid hormone synthesis.
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11. Tuesday, February 20, 2018

12. OXIDATION OF IODIDES.
 Once inside gland iodide is taken into lumen by
transporter – Pendrin.
 Then iodide is oxidised to Iodine by enzyme
Peroxidase.
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13. ORGANIFICATION OF
THYROGLOBULINS.
 Iodination of tyrosine
residue in thyroglobulin
molecule.
 Tyrosine is 1st
iodinated
at position 3 to form
Mono-iodotyrosine
(MIT) & then at position
5 to form Di-
iodotyrosine (DIT)
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14. COUPLING REACTION.
 One molecule of MIT &
DIT coupled to form
Tri-iodothyronine
(T3)
 2 molecules of DIT
coupled to form
Thyroxine (T4)
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15. STORAGE.
 Once Thyroglobulin is
iodinated is stored in
lumen.
 Stored thyroid
hormones can meet
body requirements for
1-3 months.
Tuesday, February 20, 2018

16. SECRETION
 Endocytosis – colloid
containing iodinated
Thyroglobulin is secreted by
Endocytosis.
 Colloid enters the cytoplasm
as colloid droplets.
 Proteolysis – colloid droplets
fuses lysosome vesicles
containing proteolytic
enzymes & releases T3,T4,
MIT & DIT.
Tuesday, February 20, 2018

17. TRANSPORT
 Bound form – most of
T3 & T4 are bound to
specific proteins.
 Thyroxine binding
Globulins – bind 70%
of T3 & T4.
 Free form – 0.5% of
T3 & 0.05% of T4.
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18. METABOLISM
 Deiodination – about 40%
T4 deiodinated into T3 by
enzyme 5’-deiodinase.
 Decarboxylation – very
few decarboxylated to form
tetraiodothyroacetic acid &
tri-iodothyroacetic acid.
 Conjugation – 15% T3 & T4
are conjugated in liver to
form glucoronides &
sulphates.
Tuesday, February 20, 2018

19. REGULATION OF THYROID
HORMONE SECRETION.
 Through Negative
Feedback
 Auto Regulation of
thyroid gland.
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20. THROUGH NEGATIVE
FEEDBACK
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21. AUTOREGULATION OF
THYROID GLAND.
 Thyroid gland
secretion regulated by
food iodine.
 Iodine content in diet
& iodine trapping are
inversely
proportional.
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22. MECHANISM OF ACTION OF
THYROID HORMONE
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23. ACTIONS OF THYROID
HORMONE
 Effect on growth & tissue
development.
 Effect on metabolic rate in general.
 Effects on metabolism.
 Carbohydrate
 Fats
 Proteins
 Vitamins
 Respiratory effects.
 Cardiovascular effects.
 Effects on nervous system
 Development
 Functioning of nervous tissue
 Effects on GIT.
 Effects on reproductive system
 Effects on other endocrine
glands.
 Effects on kidney.
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24. EFFECT ON GROWTH & TISSUE
DEVELOPMENT.
 Role in normal body
growth & skeletal
maturation.
 Bone development
 Teeth development.
 Role in tissue
differentiation &
maturation.
 Role in development of
nervous tissue.
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25. EFFECT ON METABOLIC RATE
IN GENERAL.
 Stimulate metabolic
activities
 Increases basal rate of
oxygen consumption
and heat production.
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26. EFFECTS ON METABOLISM.
 Carbohydrate – increase glucose absorption & metabolism.
 Fats – Mobilization of fats from adipose tissue 7 decreases
plasma cholesterol levels.
 Proteins – Physiologically anabolic effect & in high conc
catabolic effect.
 Vitamins – increases quantity of enzymes so causes vitamin
deficiency.
Tuesday, February 20, 2018

27. RESPIRATORY EFFECTS.
 Increase in resting
respiratory rate, minute
ventilation &
ventilatory response to
hypercapnia & hypoxia.
 Increase in oxygen
carrying capacity of
blood.
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28. CARDIOVASCULAR EFFECTS.
 Tachycardia.
 Increase force of cardiac
contraction.
 Increase cardiac output.
 Blood pressure – systolic
increases & diastolic
decreases.
 Vasodilatation & increased
blood flow to tissue.
Tuesday, February 20, 2018

29. EFFECTS ON NERVOUS SYSTEM
 Development
Functioning of
nervous tissue.
 Effect on functioning
of nervous tissue.
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30. EFFECTS ON GIT.
 Increase in appetite &
food intake.
 Increase in rate of
secretion of digestive
juices.
 Increase in motility of
GIT.
Tuesday, February 20, 2018

31. EFFECTS ON REPRODUCTIVE
SYSTEM
 In Males – lack of
hormones causes loss
of libido & excess
causes Impotence.
 In Females –
menorrhagia &
Polymenorrhagia.
 Irregular periods &
Amenorrhoea.
Tuesday, February 20, 2018

32. EFFECTS ON OTHER
ENDOCRINE GLANDS.
 Pituitary,
adrenocortical
secretion & oestrogen :
androgen ratio
increased.
 Parathyroid & 1,25-
(OH)2 – Vit D decreased.
Tuesday, February 20, 2018

33. EFFECTS ON KIDNEY.
 Renal plasma flow
 GFR
 Tubular transport
maximum.
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34. APPLIED ASPECTS
 Hyperthyroidism
Hypothyroidism.
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35. HYPERTHYROIDISM
 Increased secretion of
Thyroid hormones.
 Causes are
 Grave’s disease
 Toxic Nodular Goitre.
Tuesday, February 20, 2018

36. GRAVE’S DISEASE
THYROTOXICOSIS.
 Aetiology
 Autoimmune disease
 Thyroid stimulating
Antibodies against TSH
receptors.
 Bind with receptors &
mimics TSH effect
 Thyroid Hyperplasia.
Tuesday, February 20, 2018

37. SIGNS & SYMPTOMS
 General features –
 BMR
 Wt loss, heat intolerance
 Goitre – swelling of thyroid gland.
 Cardiovascular features –
 Sinus tachycardia
 Arrhythmia
 Neuromuscular features
 Nervousness, Irritability,
restlessness, Psychosis, tremors,
Muscle weakness.
Tuesday, February 20, 2018

38. SIGNS & SYMPTOMS
 Gastrointestinal – Diarrhea,
steatorrhoea & vomiting
 Dermatological –
Perspiration, loss of hair,
redness of palm.
 Reproductive
 Males – Impotence
 Females – Amenorrhoea,
Oligomenorrhoea, abortion &
Infertility.
 Ophthalmological –
Exopthalmos.
Tuesday, February 20, 2018

39. INVESTIGATIONS
 Both T3, T4 – raised.
 TSH – low
 I131
uptake –
 TRs antibodies –
 Serum Cholesterol – less
 ECG – Tachycardia, arrythmia.
 USG – Diffuse Goitre.
Tuesday, February 20, 2018

40. HYPOTHYROIDISM.
 Syndrome by low levels
of thyroid hormones.
 Aetiology –
 Primary – Thyroid
disorder.
 Secondary –
Hypothalamus or Ant
Pituitary disorder.
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41. CLINICAL FEATURES
 Infantile
Hypothyroidism
( Cretinism) –
 Adult
Hypothyroidism
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42. INFANTILE HYPOTHYROIDISM
( CRETINISM) –
 Hypothyroidism in 1st
yr
of life.
 Mental retardation,
marked retardation of
growth, delayed
milestone development,
pot belly, protruding
tongue, flat nose, dry
skin & sparse hairs.
Tuesday, February 20, 2018

43. ADULT HYPOTHYROIDISM
(MYXODEMA)
 Due to infiltration of skin
by Myxodematous
tissue.
 Clinical features –
 General – Tiredness,
weight gain, cold
intolerance, decreased
sweating.
 CVS – Bradycardia,
Anaemia.
Tuesday, February 20, 2018

44. ADULT HYPOTHYROIDISM
(MYXODEMA)
 Clinical features –
 Neurological Features – lethargy,
slow movements, speech,
sleepiness, delayed reflexes.
 Skin – toad skin, sparse hair,
myxoedema
 Reproduction – Menorrhagia,
infertility, Galactorrhoea &
Impotence.
 GIT – constipation & Adynamic
Ileus.
Tuesday, February 20, 2018

45. GOITRE
 Abnormal increase in thyroid
gland.
 Goitrogenic – causes Thyroid
enlargement.
 Thiocynates, perchlorates, thiourea,
thiouracil
 Plants – Cabbage, cauliflower.
 Iodine deficiency / endemic –
daily intake below 10 μg
Tuesday, February 20, 2018

46. THYROID FUNCTION TESTS
 Measurement of
basal metabolic rate.
 Normal BMR – ±20%
 Hyperthyroidism –
100%
 Hypothyroidism - -30
to -40%
 Radioactive iodine
uptake (RAIU) - by I131
Tuesday, February 20, 2018

47. THYROID FUNCTION TESTS
 Normal value 20-40%
 Hyperthyroidism –
60%
 Hypothyroidism –
20%
 Measurement of Total
& Free T3, T4 & TSH
 USG thyroid gland
 Thyroid scan
Tuesday, February 20, 2018

48. THYROID FUNCTION TESTS
 Anti thyroid
antibodies.
 Fine needle aspiration
Biopsy.
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49. NORMAL VALUES OF T3, T4,
TSH
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50. ANTITHYROID DRUGS
 Drugs inhibiting
iodine trapping by
Thyroid.
 Drugs inhibiting
oxidation of iodide &
coupling.
 Drugs inhibiting
release of Thyroxine
(T3,T4)
Tuesday, February 20, 2018

51. THANK
YOU