DISORDERS OF PARATHYROID GLANDS
Disorders of parathyroid
glands are of two types:
I. Hypoparathyroidism
II. Hyperparathyroidism.
HYPOPARATHYROIDISM –
HYPOCALCEMIA
Hyposecretion of PTH is
called hypoparathyroidism. It leads to hypocalcemia (decrease in blood calcium
level).
Causes for
Hypoparathyroidism
1. Surgical removal of
parathyroid glands (parathyroidectomy)
2. Removal of parathyroid
glands during surgical removal of thyroid gland (thyroidectomy)
3. Autoimmune disease
4.
Deficiency of receptors for PTH in the target cells. In this,
the PTH secretion is normal or increased
but the hormone cannot act
on the target cells. This condition is called pseudohypoparathyroidism.
Hypocalcemia and
Tetany
Hypoparathyroidism leads
to hypocalcemia, by decreasing the resorption of calcium from bones.
Hypocalcemia causes neuromuscular hyperexcitability, resulting in hypocalcemic
tetany. Normally, tetany occurs when plasma calcium level falls below 6 mg/dL
from its normal value of 9.4 mg/dL.
Hypocalcemic Tetany
Tetany is an abnormal condition
characterized by violent and painful muscular spasm (spasm =
involuntary muscular contraction), particularly in feet and hand. It is because
of hyperexcitability of nerves and skeletal muscles due to
calcium deficiency.
Signs and symptoms of
hypocalcemic tetany:
1. Hyper-reflexia and convulsions
Increase
in neural excitability results in hyper-reflexia (overactive reflex actions)
and convulsive muscular
contractions.
2. Carpopedal spasm
Carpopedal spasm is the spasm
in hand and feet that occurs in hypocalcemic tetany. During spasm, the hand shows
a peculiar attitude.
Attitude of hand in
carpopedal spasm includes:
i. Flexion at wrist joint
ii. Flexion at
metacarpophalangeal joints
iii. Extension at
interphalangeal joints
iv. Adduction of thumb.
3. Laryngeal stridor
Stridor means
noisy breathing. Laryngeal stridor means a loud crowing sound during
inspiration, which occurs mainly due to laryngospasm (involuntary
contraction of laryngeal muscles). Laryngeal stridor is a common dangerous
feature of hypocalcemic tetany.
4. Cardiovascular
changes
i. Dilatation of the heart
ii. Prolonged duration of
ST segment and QT interval in ECG
iii. Arrhythmias
(irregular heartbeat)
iv. Hypotension
v. Heart failure.
5. Other features
i. Decreased permeability
of the cell membrane
ii. Dry skin with brittle
nails
iii. Hair loss
iv. Grand mal, petit mal
or other seizures
v. Signs of mental
retardation in children or
dementia in adults.
When the calcium level
falls below 4 mg/dL, it becomes fatal. During such severe hypocalcemic
conditions, tetany occurs so quickly that a person develops spasm of different
groups of muscles in the body. Worst affected are the laryngeal and bronchial
muscles which develop respiratory arrest, resulting in death.
Latent Tetany
Latent tetany, also known
as subclinical tetany is the neuromuscular
hyperexcitability due to hypocalcemia that develops before the onset of tetany. It
is characterized by general weakness and
cramps in feet and hand. Hyperexcitability in
these patients is detected by some signs,
which do not appear in normal persons.
1. Trousseau sign
Trousseau sign is the spasm of the
hand that is developed after 3 minutes of arresting the blood
flow
to lower arm and hand.
The blood flow to lower arm and hand is arrested by inflating the blood
pressure cuff 20
mm Hg above the patient’s systolic pressure.
2. Chvostek sign
Chvostek sign is the twitch of the
facial muscles, caused by a gentle tap over the facial nerve in front of the
ear. It is due to the hyperirritability of facial nerve.
3. Erb sign
Hyperexcitability of the skeletal
muscles even to a mild electrical stimulus is called Erb sign. It is also
called
Erb-Westphal sign.
HYPERPARATHYROIDISM –
HYPERCALCEMIA
Hypersecretion of PTH is
called hyperparathyroidism.
It results in
hypercalcemia. Hyperparathyroidism is of
three types:
1.
Primary hyperparathyroidism
Primary hyperparathyroidism
is due to the development of tumor in one or more parathyroid glands.
Sometimes, tumor may develop in all the four glands.
2. Secondary
hyperparathyroidism
Secondary hyperparathyroidism
is due to the physiological compensatory hypertrophy of parathyroid
glands, in response to
hypocalcemia which occurs due to other pathological conditions such as:
i. Chronic renal failure
ii.
Vitamin D deficiency
iii. Rickets.
3. Tertiary
hyperparathyroidism
Tertiary
hyperparathyroidism is due to hyperplasia (abnormal increase in the
number of cells) of all
the parathyroid glands
that develops due to chronic secondary hyperparathyroidism.
Hypercalcemia
Hypercalcemia is the increase
in plasma calcium level. It occurs in hyperparathyroidism because of increased resorption
of calcium from bones.
Signs and
symptoms of hypercalcemia
i. Depression of the
nervous system
ii.
Sluggishness of reflex activities
iii. Reduced ST segment
and QT interval in ECG
iv. Lack of appetite
v. Constipation.
Depressive effects of
hypercalcemia are noticed when the blood calcium level increases to 12 mg/dL.
The condition becomes
severe with 15 mg/dL and it becomes lethal when blood calcium level reaches
17 mg/dL.
Other
effects of hypercalcemia:
i. Development of bone
diseases such as osteitis fibrosa cystica
ii. Development of
parathyroid poisoning. It is the condition characterized by severe
manifestations
that occur when blood
calcium level rises above 15 mg/dL. In hyperparathyroidism, the
concentration of both
calcium and phosphate increases leading to formation of calciumphosphate
crystals. Concentration of
phosphate also increases because, kidney cannot excrete the excess amount of
phosphate resorbed from the bone
iii. Deposition of calcium-phosphate
crystals in renal tubules, thyroid gland, alveoli of lungs, gastric
mucosa and in the wall of
the arteries, resulting in dysfunction of these organs. Renal
stones are
formed when it is
deposited in kidney.
PARATHYROID FUNCTION
TESTS
1. Measurement of blood
calcium level
2. Chvostek sign and
Trousseau sign for hypoparathyroidism.
CALCITONIN
Source of Secretion
Calcitonin is secreted by
the parafollicular cells or clear
cells (C cells), situated amongst the follicles in
thyroid gland. In lower animals,
the parafollicular cells are derived from ultimobranchial glands, which develop
from fifth pharyngeal pouches. In human being, the ultimobranchial glands and fifth pharyngeal pouches are
rudimentary and their cells are incorporated with fourth pharyngeal pouches and
distributed amongst the follicles of thyroid gland. Recently, calcitonin is
found in brain, prostate and bronchial cells of lungs. However, the
physiological role of calcitonin from non-thyroid tissues is not known
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