The thyroid gland is the largest endocrine gland located in the neck. It produces thyroid hormones such as T4 and T3 that regulate metabolism. The thyroid follicles contain colloid made of thyroglobulin, which iodine is attached to in order to produce the hormones. The hormones are then released into circulation and have widespread effects increasing the basal metabolic rate and promoting growth and development. Thyroid hormone production is regulated by TSH from the pituitary gland in a negative feedback loop. Disorders can result from too much or too little thyroid hormone production and affect many body systems.

1. Thyroid Gland

2. • Thyreos :
Shield

4. • Largest Endocrine gland
• Follicles 50-500μm in diameter(40 follicles)
• Colloid : Thyroglobulin (6,60,000)
• Highly Vascular : 5ml/g/min
• Functions of Thyroid Cells :
• Collection of iodine
• Thyroglobulin synthesis
• Remove Thyroid Hormones

6. Colloid
• Thyroglobulin containing iodine
• MW 6,60,000
• Function of Thyroid Cells

7. Thyroid hormones
• T4 : 93%
• T3 : 7%
• T4 converted to
T3 in target
tissues
• T3 is more potent
than T4
• Reverse T3
(3’,5’,3,–
triiodothyronine)
is inactive

8. Iodine Metabolism
• Total I- In ECF :
600 μg (500+60+40
20% : thyroid
80% in urine

9. Thyroid hormone synthesis

10. • Thyroglobulin contains more
than 70 tyrosine molecules
• Iodination of tyrosine takes
place at the apical border of
the thyroid cells while the
molecules are bound in
peptide linkage in
thyroglobulin.
Thyroid hormone
synthesis

11. • Cuboidal epithelial cells of follicle helps to
Collect (Na+/I– symporter) & transport
iodine . Synthesize thyroglobulin & secrete
it into colloid
• Remove thyroid hormones from
thyroglobulin & secrete them into
circulation
Thyroid hormone synthesis

12. Iodine trapping by epithelial cells → Oxidation of iodide
ion by peroxidase enzyme → Iodination (organification) of
tyrosine in thyroglobulin present in colloid → Storage of
thyroglobulin → Pinocytic uptake of thyroglobulin by
epithelial cells → Fusion of lysosome with pinocytic
vesicle → Cleavage of T4 & T3 from thyroglobulin →
Release of T4 & T3 (the MIT & DIT are not released & they
are deiodinated by deiodinase enzyme & iodine is reused)
→ Transported by binding to plasma proteins
Thyroid hormone synthesis

13. Distribution of Iodinated Compounds
• MIT : 25%
• DIT : 35%
• T3 : 5%
• T4 : 35%
• Secretion T3 : 4 μg/day
T4 : 80 μg/day

14. Secretion & Interconversion of Thyroid
Hormones in Normal Adult Humans

15. Regulation of Thyroid Hormone
Synthesis

16. Binding of Thyroid Hormones

17. Metabolism
• T4 Deiodinated RT3
• T4 Decarcoxylation TETRAC
Decarcoxylation
• T3 TRIAC

18. Actions of
thyroid
hormones
• Generally cause
increased BMR
• It increases
transcription of large
number of genes
• Thyroid hormone
receptors are either
attached to DNA or
located near to it

19. • Thyroid hormone increase cellular metabolic activity
– Increase the number & activity of mitochondria
– Increase active transport of ions through cell membrane
• Thyroid hormone have general &
specific effects on growth
– Metamorphic change in tadpole
– Growth & development of brain
(fetus & baby)

20. Effects on Metabolism
• Stimulation of carbohydrate metabolism
– ↑ Glucose uptake
– ↑ Glycolysis
– ↑ Gluconeogenesis
– ↑ Absorption from GI tract
– ↑ Insulin secretion
• Stimulation of fat metabolism
– ↑ Lipid mobilization from fat tissues
– ↑ Free fatty acid concentration in plasma
– ↑ Oxidation of free fatty acids by cells
• Stimulation of protein metabolism
– ↑ Protein synthesis (promotes catabolism in hyper condition)

21. Effect of Starvation

22. • Effects on various systems
• CVS
– ↑ Blood flow & cardiac output
– ↑ Heart rate
– ↑ Heart strength
– Normal arterial pressure
– High Output Cardiac Failure
• ↑ GI motility
• ↑ Respiration

23. • On Bone Marrow Metabolism(T4 deficiency
–normocytic normochromic Anaemia, megaloblastic
anaemia )
• On Vitamins- Vitamin deficiency, Beta-Carotenemia
• On Lactation- Galactopoesis
• On Gonads- poor gonadal development, Essential for
normal menstrual cycles
• Effect on CNS &PNS Myxoedema madness
• Growth &Development

24. • Effects on muscle function
–Excess make muscle week
–Hypo make muscle sluggish (relax slowly
after a contraction)
• Excess hormone cause constant
tiredness, but difficulty in sleep
• Water & Mineral Metabolism

25. Catecholamines T4
BMR, stimulates CNS,
HR& force of Contraction ,
action brief & rapid
Same action, but
prolonged
Cannot increase BMR in
absence of T4
T4 potentiate action of
Catecholamines, & in their
presence increase in BMR
is more
Stimulation of RAS

26. TSH
• 31,000, 211aa
• Half life - 60min
• 2 subunits
• Secretion - 110 μgm/day
• Plasma level - 2.3 μIU/day
• Mechanism of Action

27. Effect of TSH on Thyroid Gland
• Promotes synthesis of thyroid
hormones
• Causes secretion of thyroid
hormones

28. Physiologic Effect of Thyroid Hormones

30. Regulation of thyroid secretion
• Negative feedback effect of thyroid
hormone
• TSH increases secretory activities of
thyroid glandular cells
– ↑ Proteolysis of thyroglobulin
– ↑ Activity of Na/I symporter
– ↑ Iodination of tyrosine
– ↑ Size (cuboidal to columnar) &
number
• TSH acts through cAMP
• Exposure to cold, excitement &
anxiety increase the secretion of
TRH

31. Goiter
• Enlargement of Thyroid gland . It does not
denote the functional state of Thyroid gland
• Goitrogens : Anti-thyroid substance that cause
thyroid enlargement
• When T3 & T4 levels decreases, TSH secretion
increased.
• Excess Endogenous TSH causes the formation
of hypertrophic thyroid gland

32. Goitrogenic Agents
• Iodide Deficiency : Iodide deficiency Goiter or
Endemic goiter
• Excess Iodide
• Monovalent ions (perchlorate &thiocyanate)
• Thiocarbamides (propylthiouracil, methimazole)
• Aminobenzene (PABA, sulphonamide)
• Vegetables of Brassicaceae Family (progoitrin,
progotrin activator)

33. Causes of Congenital Hypothyroidism

34. • Hypothyroidism
• Myxoedema in Adults
1. Goiter
2.Puffness of face
3.Coarsening & Loss of Scalp hair
4.Ptosis
5. Dry, thickened rough & yellow skin
6. Low BMR, hypersensitive to cold, low
voltage ECG, hoarseness of voice,
psychosis, memory loss , increase in S.
Cholesterol
Disorders of thyroid hormone

35. Cretinism
• Due to maternal iodine deficiency & conge-
nital abnormalities of foetal hypothalamo-
pituitary-thyroid axis
1. Gross retardation of Mental Development
2. Dwarfism & Stunted growth
3. Protruded abdomen & Tongue
4. Failure of Sexual Development
5. Flat nose, dry skin, sparse hairs
6. Other Characteristic Features

36. • At adolescense ; short stature, poor
performance at school, delayed puberty, &
sexual maturation.

37. • HASHIMOTO’S THYROIDITIS

38. Hypoth--

39. Hyperthyroidism
• Results from T3 & T4
• Graves Disease
• Autoimmune Disorder
(TSA or LATS & LATS
protectors :
immunoglobulins of IgG
class)

40. Causes of Hyperthyroidism

41. • Exophthalmos
• Lid Retraction
• ↑ Excitability
• Intolerance to heat
• ↑ Sweating
• Weight loss
• Muscle weekness
• Extreme fatigue, but inability to sleep

42. Hyperthyroid-----

43. Antithyroid Drugs
• Drugs inhibit trapping of iodide
• Thiourylenes
• Iodine or Iodide
• β adrenergic blocking drugs
(propanolol, atenolol reduce CVS CNS
symptoms)
• Radioactive iodine 131I (5μ curies)

44. Hypothyroidism Hyperthyroidism
Definition T3 T4
secretion
T3 T4
secretion
Causes Thyroid removal Graves Disease
Primary I2 deficiency
/Hashimotos
Thyroid tumors
Secondary Pitu/Hypothalam
ic Failure
TSH secreting
Tumors
BMR ( N: 10%) -30 to 40% +10 – 100%
Sensitivity Hypersensitive
to cold
Hypersensitive
to heat

45. Protein
metabolism
Myxoedema Thyrotoxic
myopathy
Fat
Metabolism
S.Cholesterol, Trigly
& Phospho
all lipids
Skin Dry, cold thickened
rough
Warm, moist &
soft
CVS Bradycardia, SV, CO,
Low voltage ECG
Tachycardia, High
output cardiac
failure, exertional
dysnoea
Vitamins Carotenemia B & C
deficiency
Bone marrow Normocytic
normochromic anemia
Increased O2
delivery

46. lactation Decreases increases
gonads Menstrual
Irregularities
Scanty Periods
CHO metabolism Low Blood
Sugar
Precipitates
DM
CNS Myxoedematous
madness
Overexcitabilty, tremors,
irritability, nervouness
Growth &
Development
Slow Rapid
GIT Constipation &
Anorexia
Hyperphagia &
Diarrhoea

47. Thyroid Hormone Resistance
• Mutations in TRα coding gene
• Resistance to peripheral tissues &
ant .pituitary gland
• Attention Deficit Hyperactivity
Disorder