with clinical case

1. Clinical Scenario
• A 65-year-old female presents to the clinic feeling tired and
fatigued all the time.
• She has also noticed an increasing problem with constipation
despite adequate fiber intake. She is frequently cold when
others are hot.
• Her skin has become dry, and she has noticed a swelling
sensation in her neck area.
• On examination she is afebrile with a pulse of 60 beats per
minute.
• She is in no acute distress and appears in good health. She has
an enlarged, nontender thyroid noted on her neck.
• Her reflexes are diminished, and her skin is dry to the touch.
• Her daily dietary chart was normal but fortified salt is missing.

2. • What is the most likely diagnosis?
• What laboratory test would you need to confirm the
diagnosis?
• What is the treatment of choice?

3. Thyroid Gland
Deependra Shrestha
MSc Biochemistry
2nd year

4. Introduction:
• Named after the thyroid cartilage
• Greek: Shield shaped
• One of the largest endocrine glands
• Produces Thyroid hormones and
Calcitonin
• Plays role in metabolism, growth
and calcium homeostasis.

5. Uniqueness
• Easily seen and palpated
• Iodine
• Stored in an extracellular site
• Peptide hormones - no cell-membrane receptors
• Nuclear receptors.

6. Anatomy

7. Anatomy:
• Brownish-red gland.
• Butterfly shape gland ( H or U shaped).
• Located anteriorly in the lower neck
• Extending from the level of the fifth cervical
vertebra down to the first thoracic vertebra.
• Two lobes are present
• Lobes of gland are attached to each other by
Isthmus.
• In some people a third “pyramidal lobe” exists,
ascending from the isthmus towards hyoid bone

8. • Each lobe is 50-60 mm long
• Its weight averages 25-30 g.
• It is slightly heavier in women.
• The gland enlarges during menstruation and
pregnancy.
• Parathyroid glands usually lie between
posterior border of thyroid gland and its
sheath
• Usually 2 on each side of the thyroid.

9. Arterial Supply:
• Highly vascular
• Main supply from :
Superior thyroid arteries
Inferior thyroid arteries
Venous Drainage:
• Superior thyroid veins drain superior poles
• Middle thyroid veins drain lateral parts
• Inferior thyroid veins drain inferior poles

10. Lymphatic drainage:
Drain to:
• Prelaryngeal LN’s, Pretracheal and Paratracheal LN’s,
Cervical LN’s
• Some drainage directly into brachio-cephalic LN’s
Innervations:
• Principal innervation of the thyroid gland derives from
the autonomic nervous system.
• Parasympathetic fibers come from the vagus nerves.
• Sympathetic fibers are distributed from the superior,
middle, and inferior ganglia of the sympathetic
trunk.

11. Histology

12. • Thyroid gland contains numbers of spherical
structures i.e. Follicles.
• Follicles are the basic functional unit of this gland
• It consists of
1. Follicular cells
2. Colloids.

13. Follicular cells:
• Follicular cells are normally cuboidal in shape
• Contain many small apical vesicles
• It transports ions
• Synthesize thyroglobulin
Colloids:
• Viscous gel consisting mostly of iodinated thyroglobulin.
• Thyroid follicles are separated from each other by
connective tissue containing capillaries and another type
of cells Parafollicular cells or C cells. These cells release
Calcitonin.

14. Physiology

15. Physiology:
Thyroid gland is responsible for:
• Homeostasis of all cells
• Cell differentiation, growth, and metabolism
These physiological functions are performed by
synthesizing two principal hormones
• Thyroxine (T4 ) and triiodothyronine (T3)

16. Biosynthesis of thyroid hormones:
Requirements:
• Iodine
• Thyroglobulin
• Thyroid Peroxidase(TPO)/Hydrogen peroxide

17. Source of Iodine:
• Available through certain foods (seafood, bread,
dairy products), iodized salt, or dietary supplements,
as a trace mineral
• The recommended minimum intake is 150 μg/day

18. Fate of iodine
• Dietary iodine (I) is main source
• Dietary iodine reaches the circulation as iodide ion, an anion.
• Iodide from plasma is actively transported by a sodium-iodine symporter.
• Then iodide oxidizes to form iodine.
• At the apical membrane of cell Pendrin protein and apical iodide
transporter (AIT) is present which mediates iodide efflux into colloid.
• Iodide accumulation in the thyroid is an active transport process that is
stimulated by thyroid stimulating hormone (TSH).
• In lacuna, iodide is oxidized (“organified”) to an iodine radical by the
thyroperoxidase (TPO) enzyme
• TPO catalyses the monoiodination, diiodination and coupling reaction
• In the follicular cells, the iodine concentration is 30- to 40-fold greater
than the circulating concentration.
• Iodide transport is inhibited by lithium, which competes with sodium.
• Antithyroid drugs such as propylthiouracil and methimazole inhibit
iodination and coupling

20. Production of Thyroglobulin:
• The follicle cells of the thyroid produce thyroglobulin
• Thyroglobulin is a very large glycoprotein.
• Thyroglobulin is released into the colloid space
• Tg is a glycoprotein homodimer of 660 kDa.
• A total of 134 tyrosine residues are found in the
homodimer, and 25 to 30 of these residues are
iodinated.
• TSH is the principal stimulator of Tg synthesis.
• Thyroid transcription factor 1 (TTF1) interacts with the
Tg promoter to stimulate Tg mRNA synthesis

22. Oxidation of Iodide and Iodination of
Thyroglobulin:
• Iodide must be oxidized to be able to iodinate tyrosyl
residues of Tg
• Iodination of the tyrosyl residues then forms
monoiodotyrosine (MIT) and diiodotyrosine (DIT), which
are then coupled to form either T3 or T4
• Both reactions are catalysed by Thyroid Peroxidase(TPO).

23. Thyroperoxidase (TPO):
• TPO catalyzes the oxidation steps involved in Iodide activation,
iodination of Tg tyrosyl residues, and coupling of iodotyrosyl
residues
• TPO has binding sites for Iodide and tyrosine
• TPO uses H2O2 as the oxidant to activate Iodide to hypoiodate (OI-),
the iodinating species

24. Proteolysis of Tg With Release of
thyroid hormones:
• T4 and T3 are synthesized and stored within the Tg
molecule
• Proteolysis is an essential step for releasing the
hormones
• To liberate T4 and T3, Tg is resorbed into the
follicular cells in the form of colloid droplets,
Pinocytosis.
• Which then fuse with lysosomes to form
phagolysosomes
• Tg is then hydrolyzed to T4 and T3, which are then
secreted into the circulation

25. Wolff-Chaikoff Effect:
• It is an auto regulatory phenomenon that inhibits
organification in the thyroid gland.
• This becomes evident secondary to elevated levels of
circulating iodide.
• It lasts several days (around 10 days),
• After which it is followed by an ‘escape phenomenon’
which is resumption of normal organification of iodine
• Escape phenomenon is believed to occur because of
decreased inorganic iodine concentration secondary to
down-regulation of sodium-iodide symporter.

26. Jod-Basedow Effect:
• Opposite of the Wolff-Chaikoff effect
• Excessive iodine loads induce hyperthyroidism
• Observed in hyperthyroid disease processes
– Graves’ disease
– Toxic multinodular goiter
– Toxic adenoma
• This effect may lead to symptomatic thyrotoxicosis in
patients who receive large iodine doses from
– Dietary changes
– Contrast administration
– Iodine containing medication ( Amiodarone )

27. Hormonal Transport:
• More than 99% of circulating T4 and T3 is bound to
plasma carrier proteins
• Thyroxine-binding globulin (TBG), binds about 75%
• Transthyretin (TTR), also called thyroxine-binding
prealbumin (TBPA), binds about 10%-15%
• Albumin binds about 7%
• High-density lipoproteins (HDL), binds about 3%
• Carrier proteins can be affected by physiologic
changes, drugs, and disease

28. Free Hormone Concept:
• Only unbound (free) hormone has metabolic activity
and physiologic effects
• Free hormone is a tiny percentage of total hormone in
plasma (about 0.03% T4; 0.3% T3)
• Although the total molar concentration of T4 is 50 times
that of T3, the concentration of free T4 is only about 3
times that of free T3.
• Only free fraction of thyroid hormone is able to cross
the plasma membrane and enter the nucleus.

29. Conversion of T4 to T3:
• T4 is the primary secretory product of the thyroid gland, which is
the only source of T4
• The thyroid secretes approximately 70-90 μg of T4 per day
• T3 is derived from 2 processes
• The total daily production rate of T3 is about 15-30 μg
• About 80% of circulating T3 comes from deiodination of T4 in
peripheral tissues
• About 20% comes from direct thyroid secretion
• Sites of T4 Conversion:
• The liver is the major extrathyroidal site for production of T3
• Some T4 to T3 conversion also occurs in the kidney.
• This conversion is possible in the presence of iodothyronine
deiodinases (D1, D2 and D3).
• D1 and D2 helps in forming T3 from T4.
• D1 and D3 helps to form inactive form rT3.

30. Thyroxine (T4)
T3 rT3
Thyronine
Deiodinase
1 & 2
Deiodinase 3
DeiodinaseDeiodinase
Thyroxine Catabolism
DIT
MIT
Deiodinase
Conjugated in liver
Hydrolysed/
reabsorbed/
excreted

31. Mechanism of Action of T3:
• T3 enters plasma membrane then to nucleus
• Specific transporters facilitate the entry into cell.
• The organic anion transporting polypeptides (OATPs) and
human monocarboxylate transporter 8 (MCT8) are active
transporters
• OATPs are specific for T4 and rT3 whereas MCT8 for T3
• In nucleus it interacts with thyroid hormone receptor (THR)
• Several alpha and beta isoforms of THR are produced.
• THR usually dimerize with the retinoid X receptors(RXRs)
• The formation of the T3-THR/DNA (thyroid hormone receptor
element) complex with recruitment of transcriptional coactivators
leads to activation of target genes
• Giving rise to mRNA and protein production.

32. Mechanism of T3 hormone

33. • 4 intranuclear T3 receptors: α1, α2, β1 and β 2;
• nonfunctional receptor: α2.
• The different forms of thyroid receptors have patterns
of expression that vary by tissue and by developmental
stage.
• The presence of multiple forms of the thyroid hormone
receptor, with tissue and stage-dependent differences in
their expression, suggests an extraordinary level of
complexity in the physiologic effects of thyroid
hormone.

35. Thyroid hormone actions:
1. Growth and development:
• Stimulates formation of proteins, which exert trophic
effects on tissues
• Increase growth and maturation of bone
• Increase tooth development and eruption
• Increase growth and maturation of epidermis, hair
follicles and nails

36. 2. Nervous System:
• Critical for normal CNS neuronal development
• Enhances wakefulness and alertness
• Enhances memory and learning capacity
• Required for normal emotional tone
• Increase speed and amplitude of peripheral nerve
reflexes

37. 3. Cardiovascular System:
• Increase heart rate
• Increase force of cardiac contractions
• Increase stroke volume
• Increase Cardiac output
• Up-regulate catecholamine receptors

38. 4. Respiratory System:
• Increase resting respiratory rate
• Increase minute ventilation
• Increase ventilatory response to hypercapnia and
hypoxia
5. Renal System:
• Increase blood flow
• Increase glomerular filtration rate

39. 6. Reproductive System:
• Required for normal follicular development and ovulation
in the female
• Required for the normal maintenance of pregnancy
• Required for normal spermatogenesis in the male

40. 7. Basal metabolic rate:
• T3 increases basal metabolic rate
• Activity of the Na+/K+ pump uses up energy, in the form of
ATP
• About 1/3rd of all ATP in the body is used by the Na+/K+
ATPase
• T3 increases the synthesis of Na+/K+ pumps, markedly
increasing ATP consumption.
8. Calorigenic effects:
• T3 increases oxygen consumption by most peripheral
tissues
• Increases body heat production

41. 9. Metabolic effect:
• Stimulates lipolysis and release of free fatty acids and
glycerol
• May leads to fall in plasma cholesterol
• Increase glucose absorption.
• Increase gluconeogenesis and glycogenolysis.
• Increase insulin breakdown.

42. Hypothalamic-Pituitary-Thyroid Axis:
• Other hormones play role in synthesis of thyroid
hormones.
• Thyroid Stimulating Hormone (TSH) helps in synthesis of
thyroid hormones.
• TSH is tropin hormone.
• Synthesis of TSH is itself controlled by other hormone
• Thyrotropin Releasing Hormone (TRH) is responsible for
production of TSH.

44. Thyrotopin Releasing Hormone:
• A tripeptide: pyroGlutamate-histidine-proline-amide
• Synthesized from a 29 kDa precursor protein
• Produced by Hypothalamus, medial neurons of the
paraventricular nucleus.

45. • Thyrotropin-releasing hormone is very short-lived, lasting
for a matter of two minutes.
• Travels less than an inch in the bloodstream to the pituitary
gland before it is broken down.
• Release is pulsatile, circadian
• Travels through portal venous system to
adenohypophysis
• Stimulates TSH formation
• Also stimulate the release of another hormone from the
pituitary gland, prolactin.

46. Influence of TRH on TSH Release:
• TRH is a hypothalamic releasing factor which travels through
the pituitary portal system to act on anterior pituitary
thyrotroph cells.
• TRH acts through G protein-coupled receptors, activating the
IP3 (Ca2+) and DAG (PKC) pathways to cause increased
production and release of TSH.
TRH phospholipase C
G protein-coupled
receptor
IP3 calcium
DAG PKC
calmodulin

47. Thyroid Stimulating Hormone:
• It is a glycoprotein hormone synthesized and secreted by
thyrotrope cells in the anterior pituitary gland.
• Its secretion follows circardian rhythm, highest between
midnight and 4 AM and lowest at midday
• Consists of two subunits, the
alpha and the beta subunit.

48. • The α (alpha) subunit is nearly identical to that of human
chorionic gonadotropin (hCG), luteinizing hormone
(LH), and follicle-stimulating hormone(FSH).
• The α subunit is thought to be the effector region
responsible for stimulation of adenylate cyclase (involved
the generation of cAMP)
• The β (beta) subunit (TSHB) is unique to TSH, and
therefore determines its receptor specificity
• The α chain has a 92-amino acid sequence.
• The β chain has a 118-amino acid sequence.

49. • Thyroid hormones exert negative feedback on TSH
release by:
• inhibition/stimulation of TSH synthesis
• decreasing/increasing pituitary receptors for TRH

50. Action of TSH on the Thyroid:
• TSH acts on follicular cells of the thyroid.
- increases iodide transport into follicular cells
- increases production and iodination of thyroglobulin
- increases endocytosis of colloid from lumen into follicular cells
Na+
I-
thyroglobulinfollicle
cell
gene
I-
endocytosis
thyroglobulin
T3 T4
colloid droplet
I-I+
iodination
thyroglobulin
Na+ K+
ATP

51. Mechanism of Action of TSH:
• TSH binds to a plasma membrane-bound, G protein-coupled
receptor on thyroid follicle cells.
• Specifically, it activates a G-coupled receptor, resulting in
increased cAMP production and PKA activation.
TSH
Gsa
Adenylyl
Cyclase
ATP cyclic AMP
Protein kinase
A
Follicle cell

52. Regulation of Thyroid Hormone
Levels:
• Thyroid hormone synthesis and secretion is
regulated by two main mechanisms:
• an “autoregulation” mechanism, which reflects the
available levels of iodine
• regulation by the hypothalamus and anterior pituitary

53. Autoregulation of Thyroid
Hormone Production:
• The rate of iodine uptake and incorporation into
thyroglobulin is influenced by the amount of iodide
available:
• low iodide levels increase iodine transport into follicular
cells
• high iodide levels decrease iodine transport into
follicular cells
• Thus, there is negative feedback regulation of
iodide transport by iodide.

54. Feedback mechanism:
• Increase and decrease in the thyroid hormone level can
itself maintain its level.
• When there is increase in Thyroid hormone level in
blood it inactivates TRH and TSH synthesis
• As a result there will be decrease production of thyroid
hormones
• And vice-versa.
• Thyroid hormones exert negative feedback on TSH
release at the level of the anterior pituitary.

55. Factors affecting Thyroid hormones:
Age:
Fetal Life:
• T4 and TSH are detectable at 10-12 weeks of gestation.
• At about 36 weeks total and free T4 reach adult level.
• However total and free T3 reach lower limit of adult range.
Neonate:
• There is rapid, transient release of TSH, T4 and T3 occurs.
• TSH levels peaks at first 30 mins, stimulates T3 and T4 production.
• This effect attenuated in premature birth.

56. Infancy and childhood:
• TSH and free T4 level are as adult range.
• Free T3 is higher than in adults.
Elderly:
• Modest decrease in T4
• Slight fall in T3 and TSH.
Pregnancy:
• There is rise in free T3 and T4 concentration due to thyroid
stimulating action of hCG.
• This lead to suppression of TSH.
• As pregnancy progress free hormone level decreases and TSH level
rise.

57. Drugs:
Mechanism Example of Drugs
Decrease in TSH secretion Dopamine, glucocorticoids, cytokines
Decrease in thyroid hormone secretion Lithium, amiodarone, iodine
Increase in thyroid hormone secretion iodine
Displacement of thyroid hormone from
plasma proteins
Furosemide, salicylates, NSAIDS
Increase hepatic metabolism Phenytoin, rifampicin, barbiturates
Impaired T4 and T3 conversion Beta antagonist, radiocontrast dye
Impaired absorption of thyroxine Calcium, sucralfate, soya protein
Modified thyroid hormone action amiodarone

58. Other factors:
• Thyroid hormone metabolism is markedly affected by fasting and
illness.
• There is release of dopamine, cortisol, somatostatin along with
cytokines which suppress TSH.
• Decrease production of T3, changes in function of T3 receptors leads
to low T3.
• Catecholamines and leptin stimulate TRH production hence regulate
TSH

59. Discussion to case
• Diagnosis: Iodine deficiency hypothyroidism (Goiter)
Iodine deficiency
Decrease synthesis of thyroid hormone
Decrease release of thyroid hormone
Stimulate pituitary to produce THS
Increase release of TSH
Increase cellularity of thyroid gland
Hyperplasia of thyroid
Goiter
Visible neck mass
Weezing and coughing
Fatigue
Dyspagia
Weight gain
pain

60. • Laboratory tests: TSH and free T4
• Treatment: Iodine supplementation on diet and
Thyroid hormone replacement with levothyroxine

61. •Thank you