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Health Tip:
Hypothyroidism. Part 2: Testing
Last week's Health Tip discussed the causes, symptoms, and treatment of hypothyroidism,
a condition characterized by abnormally low thyroid hormone production. Today's
tip will describe how hypothyroidism is diagnosed.
A brief discussion of thyroid gland physiology will help in understanding the tests
that are used in diagnosing hypothyroidism.
Normal thyroid hormone production. The
thyroid gland produces two main hormones, thyroxine (T-4) and triiodothyronine (T-3).They control a number of functions in the
body including the rate at which the body uses fats and carbohydrates, body temperature,
and to some extent how fast the heart beats. T-3 and T-4 production is controlled
by a "feedback loop" mechanism which involves the thyroid gland, and two structures
within the brain, the hypothalamus and the pituitary gland. When the hypothalamus
senses that the body needs more thyroid hormone, it signals to the pea-sized pituitary
gland to produce a hormone known as TSH (thyroid stimulating hormone). Under the
influence of TSH, the thyroid gland releases thyroid hormone into the bloodstream.
With adequate amounts of thyroid hormone in the bloodstream, the pituitary temporarily
suspends secretion of TSH and thyroid hormone production is suspended. With additional
need for thyroid hormone by the body, this mechanism is again activated.
What happens
to thyroid hormone production in hypothyroidism? If, for one of the reasons mentioned
in the previous Health Tip, the thyroid is unable to produce thyroid hormone, circulating
T-3 and T-4 levels drop. At that point, the pituitary begins to release more and
more TSH in an effort to stimulate thyroid hormone production. Due to the diseased
state of the thyroid gland, however, the additional TSH is unable to further stimulate
thyroid hormone production.
How is hypothyroidism diagnosed? The diagnosis of hypothyroidism
can be suspected in patients with the classic hypothyroid symptoms of fatigue, constipation,
dry skin, thinning hair, and cold intolerance. Blood tests, however, are needed
to confirm the diagnosis. The primary test used for the detection of hypothyroidism
is the measurement of the TSH. An elevated TSH level represents an important hallmark
in diagnosing hypothyroidism. The T-3 and T-4 levels can also be measured directly,
and this is often done to further confirm the diagnosis of hypothyroidism.
Other
roles for the TSH tests. TSH testing is also used to help diagnose an asymptomatic
condition called "subclinical hypothyroidism". In this condition, the blood levels
of T-3 and T-4 are normal but the TSH is elevated. Over time, people with subclinical
hypothyroidism will most likely develop overt hypothyroidism. TSH testing also plays
an important role in managing hypothyroidism. After thyroid hormone replacement
has been started, a TSH level can be obtained periodically to make sure that someone
is taking the correct dosage. With adequate thyroid hormone replacement, the TSH
should be in the normal range. Lastly, since hypothyroidism is so common among the
elderly, the TSH may also be used to screen for asymptomatic hypothyroidism in those
individuals at risk.
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