Thyroid Blood Tests
Medical Author: Ruchi Mathur, M.D.
Medical Editor:
William C. Shiel Jr, MD, FACP, FACR
Introduction
The thyroid gland produces hormones that are essential for normal body metabolism.
Blood testing is now commonly available to determine the adequacy of the levels of
thyroid hormones. These blood tests can define whether the thyroid gland's hormone production is normal, overactive, or
underactive.
What are thyroid hormones?
Thyroid hormones are produced by the thyroid gland. This gland is located in the lower
part of the neck, below the Adam's apple. The gland wraps around the windpipe
(trachea) and has a shape that is similar to a butterfly - formed by two wings (lobes) and attached by a middle part (isthmus).
The thyroid gland uses iodine (mostly available from the
diet in foods such as seafood, bread, and salt) to produce thyroid hormones. The two most important
thyroid hormones are
thyroxine (T4)
and
triiodothyronine (T3), which account
for 99.9% and 0.1% of thyroid hormones present in the blood respectively.
However, the hormone with the most biological activity is T3. Once released from
the thyroid gland into the blood, a large amount of T4 is converted into T3 -
the active hormone that affects the metabolism of cells.
Thyroid hormone regulation -- the chain of command
The thyroid itself is regulated by another gland that is located in the brain, called the pituitary.
In turn, the pituitary is regulated in part by the thyroid (via a "feedback" effect of thyroid hormone
on the pituitary gland) and by another gland called the hypothalamus.
The hypothalamus releases a hormone called thyrotropin releasing hormone (TRH), which sends a signal
to the pituitary to release thyroid stimulating hormone (TSH). In
turn, TSH sends a signal to the thyroid to release thyroid hormones. If a disruption occurs at any of
these levels, a defect in thyroid hormone production may result in a deficiency of thyroid hormone
(hypothyroidism).
Hypothalamus - TRH
Pituitary- TSH
Thyroid- T4 and T3
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The rate of thyroid hormone production is controlled by the pituitary gland. If there
is an insufficient amount of thyroid hormone circulating in the body to allow for normal
functioning, the release of TSH is increased by the pituitary gland in an attempt to stimulate
more thyroid hormone production. In contrast, when there is an excessive amount of circulating
thyroid hormone, TSH levels fall as the pituitary attempts to decrease the production of thyroid
hormone. In persons with hypothyroidism (thyroid hormone production is below normal), there is a
continuously decreased level of circulating thyroid hormones. In persons with hyperthyroidism (thyroid
hormone production is above normal), there is a continuously elevated level of circulating thyroid hormones.
Next: How is hypothyroidism diagnosed? »
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