Thyroid GlandThyroid Gland

DR SYED SHAHID HABIBDR SYED SHAHID HABIB

MBBS FCPS DSDMMBBS FCPS DSDM

Thyroid GlandThyroid Gland

• HistologyAcini / follicles ( active & inactive gland)

Cells

Thyroid cells –

amine hormones- T3,T4, and rT3

C - cells (Para follicular cells)

protein hormone – Calcitonin)

Thyroid HormonesThyroid Hormones

• Triiodothyronine (T3) By Thyroid Follicular Cells

• Thyroxine or Tetraiodothyronine (T4) By Thyroid Follicular Cells

• Reverse Triiodothyronine - rT3 By Thyroid Follicular Cells

• Calcitonin By Thyroid Para Follicular C Cells

SYNTHESISSYNTHESIS

ColloidBloodThyroid

Cell

I-

ATP

Na

I- I

MIT

Tyrosine

Na I

DIT

Thyroglobulin Mol

MIT DIT

DITDIT

T3

T4

Peroxidase

TGPeroxi

EPRExo

++

IODIDE TRAPPINGOXIDATION OF

IODIDE ION

IODINATION OF TYROSINE

COUPLING

SECRETIONSECRETION

Colloid Blood Thyroid

Cell

DIT

Thyroglobulin Mol

MIT DIT

DITDIT

T3

T4

MIT DIT

T3 T4

Lysopro

MIT DIT

T3

T4

I Tyrosine

Deiodinase

I

Endo

TSH FROM ANTERIOR PITUITARY

Production of Thyroid Production of Thyroid HormonesHormones

• Iodide (I-) actively transported into the follicle and secreted into the colloid.

• Oxidized to iodine (Io).• Iodine attached to tyrosine within

thyroglobulin chain.– Attachment of 1 iodine produces

monoiodotyrosine (MIT).– Attachment of 2 iodines produces

diiodotyrosine (DIT).

• MIT and DIT or 2 DIT molecules coupled together.

Production of Thyroid Hormones Production of Thyroid Hormones (continued)(continued)

• T3 and T4 produced.

• TSH stimulates pinocytosis into the follicular cell.– Enzymes hydrolyze T3 and T4 from

thyroglobulin.

• Attached to TBG and released into blood.

Production of Thyroid Hormones Production of Thyroid Hormones (continued)(continued)

T3 T4Normal Plasma level 1.2 – 3.1 n mol/L 60-160 n mol/L

Free 0.3 ng 0.15 ug

Bound 99.8% 99.98%

Binding Proteins Thyroxin Binding Globulin

Thyroxin Binding Globulin

Duration of action 1-2 days 4-6 days

Site of formation/ Source

Thyroid Cells + T4 Thyroid Cells

Potency 4-5 times More potent Less potent

Mechanism of ActionMechanism of Action

T3,T4 – Genomic action – formation of new proteins

TRH – Activation of Phospholipase-C System

TSH – Activation of cAMP system

FEEDBACK FEEDBACK CONTROL CONTROL

OF THYROIDOF THYROIDSECRETIONSECRETION

T3 T4T3 T4

Pituitary Pituitary TSHTSH

HYPOTHALAMUSHYPOTHALAMUS

TRHTRH

PHYSIOLOGICAL EFFECTS OF PHYSIOLOGICAL EFFECTS OF THYROID HORMONETHYROID HORMONE

Regulation of metabolism – (O2 consumption)

Mental development Normal growth

potentiate somatomedin Normal secretion of GH.

Skeletal maturation ossification of cartilage & growth of teeth

Normal menstrual cycle Conversion of carotene to vitamin A.

Actions of TActions of T33

• Stimulates protein synthesis.• Promotes maturation of nervous system.• Stimulates rate of cellular respiration by:

– Production of uncoupling proteins.– Increase active transport by Na+/K+ pumps.– Lower cellular [ATP].

• Increases metabolic heat.• Increases metabolic rate.

– Stimulates increased consumption of glucose, fatty acids and other molecules.

Effects of Hypersecretion Effects of Hypersecretion

• Effects of hypersecretion are due to

increased O2 consumption

Catabolism of carbo; proteins; & fats.

Enzyme synthesis

No. and size of mitochondria

ATPase activity

Effects of HypersecretionEffects of Hypersecretion

• Caloregenic Effects

• Effects on CVS

• Effects on CNS

• Effects on GIT

• Effects on Sympathetic system

• Effects on Growth

Caloregenic effectsCaloregenic effects

• It involves the effects on – Carbohydrate Metabolism– Protein Metabolism– Fat Metabolism

Increased Catabolism

EFFECTSEFFECTS ON CVSON CVS

Increases • Heart rate

• Cardiac output

• Systolic pressure

• Pulse pressure

• Mean pressure

Effects on CNSEffects on CNS

Rapid cerebration Psychoneurotic tendency Tremors Time for stretch reflex is reduced Decreased sleep

Table 11.8Table 11.8

GOITER

Features of HyperthyroidismFeatures of Hyperthyroidism

• Increased – BMR, and Heat production

– Heart rate – palpitation

– Weight loss

Diarrhea

Exophthalmos & corneal ulcers

GRAVE’S DISEASE

Features of HypothyroidismFeatures of Hypothyroidism

• Decreased BMR and Pulse rate• Easy fatigability• Slow reflexes• Slow mentation• Slow speech• Increased weight• Increased sleep• Myxedema (water, chondriatin sulphate, and

hyaluronic acid)

Cretinism