ACTIONS OF THYROID
HORMONE
Dr. Ayisha Qureshi
Assistant Professor
MBBS, MPhil
1. GENES
•
Thyroid hormone increases the transcription of large
number of genes.
↓
Thus, in all cells of the body, there is increased production
of:
1. Protein enzymes
2. Structural proteins
3. Transport proteins
↓
Net result: there is generalized increase in functional
activity throughout the body!
2. CALORIGENIC ACTION
Increases the O2 consumption of almost all metabolically active
tissues + increases activity of the Na-K Pump
↓
Increased metabolic rate (BASAL METABOLIC RATE)
(Rate of O2 consumption & energy expenditure under resting
conditions)
↓
Increased Calorigenic effect (increased heat production)
↓
If food intake is not increased, endogenous protein and fat stores
are catabolized
↓
Weight loss occurs
3. GROWTH & MATURATION
GROWTH & MATURATION
a: Skeletal System
• At the level of bone formation, thyroid acts
synergistically & permissively with GH, IGF-1 &
other growth factors.
• It is required for normal growth hormone
synthesis & secretion.
• Thyroid hormone accelerates bone
maturation. (skeletal maturation is different from
skeletal growth!)
• Bone age is, thus, retarded relative to
chronological age in children who are deficient in
thyroid hormone.
• In cases that are Hyperthyroid------------------?
GROWTH & MATURATION
b: Central Nervous System
• Thyroid hormone receptors are present early in the
development of the fetal brain, well before the thyroid gland
becomes functional.
• Maturation of the nervous system during the Perinatal period
has an absolute dependence on thyroid hormone.
↓
During this period, Thyroid hormone must be present for normal
brain development.
↓
It is especially needed for nerve myelination, brain vascularity &
normal brain growth!
↓
Thyroid hormone promotes growth & development of the brain
during fetal life & for the first few years of postnatal life.
GROWTH & MATURATION
b: Central Nervous System
• If thyroid hormone conc. insufficient both before & after
birth, the growth & maturation of the brain are greatly
reduced.
↓
Brain remains smaller! (due to decrease in axonal density
& dendritic branching & vascularity)
• Without specific thyroid hormone therapy within days or
weeks after birth, the child without a thyroid gland will
remain mentally deficient throughout life!
↓
Thus, thyroid hormone deficiency at this stage can be
catastrophic & the damage done is irreversible even if
large doses of the hormone are given later in childhood!
GROWTH & MATURATION
b: Central Nervous System
•
1.
2.
3.
4.
In Adult life: Hyperthyroidism produces:
Hyper excitability
Irritability
Restlessness
Exaggerated responses to environmental
stimuli
5. Emotional instability
6. Full-blown Psychosis
GROWTH & MATURATION
b: Central Nervous System
•
1.
2.
3.
4.
5.
6.
7.
In Adult life: Hypothyroidism produces:
Listlessness
Lack of energy
Slowness of speech
Impaired memory
Somnolence
Dulled mental capacity
Psychosis (myxedema madness)
GROWTH & MATURATION
POINTS TO REMEMBER:
• It stimulates growth of the skeletal system.
• It stimulates normal synthesis & secretion of the GH.
• It stimulates & promotes normal growth & development
of the brain during the perinatal period.
• Its deficiency during postnatal period can lead to
irreversible mental retardation & small sized brain.
• In the adult, it has excitatory effects on the nervous
system.
ACTION OF THYROID
HORMONE
3. AUTONOMIC NERVOUS
SYSTEM
SYMPATHOMIMETIC EFFECTS
Any action similar to one produced by the sympathetic
nervous system is called a Sympathomimetic effect.
• Interactions b/w thyroid hormone & ANS are
important throughout life.
• ↑ secretion of thyroid hormone exaggerates many
responses of the sympathetic neurons.
• It ↑ the number of receptors for epinephrine & NE
(beta- adrenergic receptors) in the myocardium &
other tissues.
• Thus, many symptoms of hyperthyroidism resemble
sympathetic nervous system.
ACTION OF THYROID
HORMONE
4. METABOLISM
a. Carbohydrate metabolism
•
1.
2.
3.
4.
5.
6.
It stimulates all aspects of carbohydrate
metabolism
↑ uptake of glucose by the cells
↑ glycolysis
↑ gluconeogenesis
↑ rate of absorption of glucose from GIT
↑ glycogenolysis
↑ secretion of Insulin
b. FATS
• It stimulates all aspects of Fat metabolism.
• POINT TO REMEMBER: Thyroid hormone
is not the primary determinant for
accelerating fat metabolism BUT is acting
as GAIN control thus it amplifies the steps
without starting them!
• It stimulates lipid mobilization from fat
tissues
• RESULT?
b. FATS
• ↑ thyroid hormone ↓ the conc. of
cholesterol, PL, and TG in the blood.
• ↓ thyroid hormone ↑ the conc. of
cholesterol, PL, and TG in the blood +
causes excessive fat deposition in the liver
• M.O.A: Increased rate of cholesterol
secretion in the bile & consequent loss in
the feces. This is done by increasing
lipoprotein receptors on the liver cells.
PROTEIN METABOLISM
1.
2.
3.
4.
• Both synthesis & degradation of proteins are
slowed in the absence of thyroid hormones.
&
accelerated by the thyroid hormones!
Increased translation of RNA
Increased transcription of DNA to RNA
Increased activity of mitochondrias
Increased activity of cellular enzymes
SO, what will happen in the HYPERTHYROID
SUBJECTS?
ACTION OF THYROID
HORMONE
5. HEART
BLOOD FLOW & CARDIAC
OUTPUT
Increased BMR
↓
Increased & more rapid oxygen consumption by the tissues
↓
Greater than normal metabolic end products
↓
Vasodilatation in most tissues
↓
Increased blood flow
↓
Increased cardiac output
HEART RATE
• Thyroid hormone causes considerably
increased heart rate---TACHYCARDIA
↓
A very sensitive symptom thru which the
clinician determines whether a patient has
excessive or diminished thyroid hormone
production
HEART STRENGTH
• Small increase in thyroid hormone secretion:
Increase in heart strength due to increased
enzymatic activity (as occurs in mild fevers
during exercise)
• Large increase in thyroid hormone secretion:
heart strength becomes depressed b/c of long
term protein metabolism
Some severely Thyrotoxic patients die of severe
cardiac decompensation (due to myocardial
failure b/c of increased cardiac output causing
increased cardiac load)
ACTION OF THYROID
HORMONE
6. RESPIRATION
Increased rate of metabolism
↓
Increased utilization of oxygen
+
Increased formation of carbon dioxide
↓
Increased rate & depth of Respiration
ACTION OF THYROID
HORMONE
7. GASTROINTESTINAL TRACT
• Increased appetite
• Increased food intake
• Increased GI motility
• Increased GI secretions
SO, Hypothyroidism causes:__________
& Hyperthyroidism causes: __________
ACTION OF THYROID
HORMONE
8. MUSCLE
MUSCLE VIGOR
• Slight increase in thyroid hormone
secretion: muscles react with vigor
• Large increase in thyroid hormone
secretion: muscles become weakened b/c
of excess protein catabolism.
WITH HYPOTHYROIDISM MUSCLES
BECOME SLUGGISH & THEY RELAX
SLOWLY AFTER A CONTRACTION!
MUSCLE TREMOR
•
•
•
•
Usually seen with hyperthyroidism
Fine muscle tremor
10-15 / sec
Observation: place a sheet of paper on
the extended fingers & noting the degree
of vibration of the paper!
• Cause: increased reactivity of neuronal
synapses
ACTIONS OF THYROID
HORMONE
9. SLEEP
SLEEP
Increased thyroid hormone secretion
↓
Exhaustive effect on the musculature & CNS
↓
Constant tiredness
BUT, b/c of the excitable effects on
synapses, IT IS DIFFICULT TO SLEEP
SLEEP
• In Hypothyroid individuals, there is
SOMNOLENCE
(Sleep sometimes lasts from 12 to 14 hours)
ACTIONS OF THYROID
HORMONE
10. OTHER ENDOCRINE
GLANDS
INSULIN
Increased rate of Glucose absorption from
the GIT and increased rate of
gluconeogenesis.
↓
Increased Blood glucose levels
↓
Increase in Insulin Secretion
PARATHYROID HORMONE
Increase in bone maturation and stimulation
of bone metabolic activities
↓
Increase in Parathyroid Hormone Secretion
GLUCOCORTICOIDS
Thyroid hormone increases the rate at which the
glucocorticoids are inactivated by the liver
↓
Feedback increase in adrenocorticotropic hormone
(ACTH) production by the anterior pituitary
↓
Increased rate of secretion of glucocorticoids by
the adrenal glands
11. Effect on Body weight
TH is essential for maintaining body weight.
• Increase in TH → Decreases the Body
weight and fat
stores.
• Decrease in TH → Increases the Body
weight and fat stores.
12. Action on Blood
TH is one of the important GENERAL
factors required for Erythropoiesis.
↓
Accelerates Erythropoietic activity leading to
an increase in the blood volume.
↓
Polycythemia is common in
Hyperthyroidism.
ACTION OF THYROID
HORMONE
13. SEXUAL FUNCTIONS
•
For normal sexual development and
function, thyroid hormone production
must be normal.
↓
This effect is exerted by:
1. A direct metabolic effect on the gonads
2. Excitatory & inhibitory feedback effects
operating through the anterior pituitary.
MALES
• Lack of thyroid hormone causes complete
loss of libido (sexual drive)
• Excess of thyroid hormone causes
impotence
FEMALES
• Hypothyroidism leads to:
- Menorrhagia (excessive bleeding)
- Polymenorrhea (frequent menstrual
bleeding)
- Irregular menstruation
- Amenorrhea
• Hyperthyroidism leads to:
- Oligomenorrhea
- Amenorrhea