Disorders of Paratharmone

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