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Thyroid secretion is controlled primarily
bythyroid-stimulatinghormone (TSH),secreted by the anterior pituitary gland.
To form normal quantities of thyroxine,
50 mg of ingested iodine are required
each year or 1 mg per week.
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STAGES IN FORMATION OF THYROID
HORMONES
1. IODIDE PUMP ( iodide trapping)
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1. OXIDATION OF THE IODIDE IONpromoted by enzyme
peroxidase
2. IODINATION OF TYROSINE AND FORMATION OF
THYROID HORMONESORGANIFICATIONIodinase enzyme
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Monoiodot rosine and diiodot rosine
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4. COUPLING
Monoiodotyrosine + diiodotyrosine = T3
Diiodotyrosine + diiodotyrosine = T4
Release of T3 and T4 into circulating bloodProteinase enzyme
Transport of T3 and T4combine with proteins
a.thyroxinebinding globulunb.thyroxinebinding pre-albumin
c.albumin
T4 released to tissues every 6 days and T3 in one day
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PHYSIOLOGIC FUNCTIONS OF
THYROID HORMONES
1. Increase the Transcription of Large Number of Cells
a. most thyroxine secreted by the thyroid is converted to
triiodothyroinine
b.thyroid hormones activate nuclear receptors
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2. Increase Cellular Metabolic Activity
a. thyroid hormone increase the number and activity of
mitochondriab.thyroid hormone increase active transport of ions
through cell membrane
3. Effect on Growthpromotes growth and development
of the brain during fetal life and few years of postnatal life
4. Effect on Specific Bodily Metabolism
a. Stimulation of Carbohydrate Metabolismb. Stimulation of Fat Metabolism
c. Increased Requirement for Vitamins
d. Increased Basal Metabolic Rate
e. Decreased Body Weight
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f. Effect on Cardiovascular system
i. Increased Blood Flow and Cardiac Output
ii. Increased Heart Rate
iii. Increased Heart Strength
iv. Normal Arterial Pressureg. Increased Respiration
h. Increased Gastrointestinal MotilityDiarrhea
i. Excitatory Effect on the Central Nervous System
Muscle Tremors
j. Effect on Sleepinsomnia (difficult /inability to sleep)
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k. Effect on Sexual Function
i. lack of thyroid hormone cause Menorrhagia(excessive menstrual bleeding ) and
Polymenorrhea (frequent menstrual bleeding)
greatlydecreased Libido
ii. Hyperthyroid womanOligomenorrhea
(greatly reduced bleeding andAmenorrhea
(absence)
iii. In menlack of thyroid hormonecauses loss oflibido ; great excess cause
impotence
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DISEASES OF THE THYROID
1 Hyperthyroidism( Toxic Goiter , Thyrotoxicosis, Graves Disease)
Symptoms of Hyperthyroidism:
a. High State of Excitability
b.Intolerance to Heat
c. Increased Sweatingd. Mild to Extreme Weight Loss
e.Diarrhea
f.Nervousness of other Psychic disorders
g. Extreme Fatigue & Inability to Sleep
h. Tremor of the Hands
i. Exophthalmosprotrusion of the eyeball
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2.HypothyroidismSymptoms opposite to that of hyperthyroidism
a. Endemic Goiter (enlarged thyroid gland)due todietary iodine deficiency
b. Constipation
c. Myxedemabagginess under the eyes and swelling of
the face
d. Arteriosclerosis-e. Somnolence
f. Increase in Weight
g.Decreased blood volume
h. Slow heart ratei. Mental sluggishness
j. Depressed growth of hair and scaliness of the skin
k. Froglike husky voice
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REGULATION OF THYROID HORMONE
SECRETION
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1. TSH or Thyrotropin ( from the Anterior Pituitary Gland)
Increases Thyroid Secretion
Effects on the thyroid gland:a. Increased proteolysis of the thyroglobulin
b. Increased activity of the iodide pump
c. Increased iodination of tyrosine
d. Increased size and increased secretory activity of thethyroid cells
e. Increased number of thyroid cells
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TSH INCREASES ALL KNOWNSECRETORY ACTIVITIES OF THE
THYROID GLAND CELLS
Cyclic Adenosine Monophosphate
Mediates the Stimulatory effect of TSH-
cAMP act as second messengerinmany target tissues
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2. Anterior Pituitary Secretion of TSH is regulated by Thyrotropin
releasing Hormone from the Hypothalamus
-Activation of the phospholipase second messenger system
Stimuli that Affect Rate of Secretion of TRH and TSH:
a. Exposure to Coldincreases secretion
b. Emotional reactions- (Excitement and Anxiety)cause decrease in secretion of TSH
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3. Feedback Effect of Thyroid Hormone toDecrease Anterior
Pituitary Secretion of TSHIncreased thyroid hormone secretion decreases
secretion of TSH
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ANTITHYROID SUBSTANCESdrugs that
suppress thyroid secretion
1. Thiocyanatedecreases iodide trapping2. Propylthiouracildecreases thyroid hormone formation
Mechanism:
a. block the proxidase enzyme required for
iodination of tyrosineb. block the coupling of two iodinated tyrosines
to form thyroxine and triiodothyronine
3. Iodidesin high concentratioin decreases thyroid activity
and thyroid gland size
- reduce the rate of iodide trapping
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CRETINISMCaused by extreme hypothyroidusm during
fetal life, infancy or childhood.
Characteristics:a. Failure to grow
b. Mental retardation
c. Skeletal growth us characteristically more
inhibited than is soft tissue growth
- child is obese, stocky and short appearance
-tongue becomes large in relation to skeletal growth
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CAUSES:
1. Congenital Cretinismresults from congenital
lack of a thyroid glandfailure of thyroid glandto produce thyroid hormone
2.Endemic Cretinismiodine lack in the diet
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