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
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
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)
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
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
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.