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CASE FILES: BIOCHEMISTRY
Secondary hyperparathyroidism arises from chronic hypocalcemia. This
condition can result from renal failure leading to poor reabsorption of calcium
from the urinary filtrate. It can also arise from poor nutrition or malabsorption
of vitamin D by the intestine. In response, parathyroid glands increase their
secretion of PTH. This condition also leads to decalcification of bone. Tertiary
hyperparathyroidism is often seen after renal transplantation. In these patients,
the parathyroid gland secretes the hormone independently of blood calcium
levels.
Paradoxically, although chronic exposure to high PTH levels leads to
bone decalcification, administration in pulses, as a once-a-day injection,
stimulates an increase in bone mass. This treatment is now used as an effec-
tive therapy for osteoporosis. Calcitonin, secreted by the parafollicular C cells
of the thyroid gland, opposes effects of PTH. Calcitonin is secreted when
blood levels of calcium are too high, and it acts to suppress reabsorption of cal-
cium in the kidney and inhibit bone demineralization. However, in humans, it
plays a minor role in regulating blood levels of calcium ion.
C O M P R E H E N SIO N Q U E ST IO N S
[51.1] A 54-year-old patient complained of muscle weakness, fatigue, and
depression. She had a recent episode of renal stones and a bone scan
revealed osteopenia. She had not yet entered menopause. She has taken
a daily multivitamin tablet plus an additional 500 mg of calcium citrate
for the past 20 years. Results from blood chemistry analysis indicated
elevated levels of serum calcium ion. Urinalysis indicated phospha-
turia. The patient’s symptoms are most likely caused by which of the
following?
A. Excessive intake of vitamin D
B. Excess synthesis of parathyroid hormone
C. Excessive intake of calcium
D. Excess synthesis of calcitonin
E. Excess intake of phosphate
[51.2] Before the introduction of vitamin D-fortified milk, children who spent
most of their time indoors often developed rickets. In these vitamin D-
deficient children, the most likely explanation for their bone malfor-
mations is which of the following?
A. Excessive renal excretion of calcium
B. Excessive renal excretion of phosphate ion
C. Inadequate uptake of calcium in the intestine
D. Lack of weight-bearing exercise
E. Excessive renal tubular reabsorption of calcium
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