Hypothyroidism
Medical Author: Ruchi Mathur, MD, FRCP(C)
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR
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Thyroid, Iodine, & Diet: What You Should Know
Medical Author: Ruchi Mathur, MD
Medical Editor: William C. Shiel, Jr, MD, FACP, FACR
 Many of my patients ask questions or make comments about iodine use in thyroid disease. Examples are: " Should I increase the iodine in my diet if I'm hypothyroid?" "My mother had hyperthyroidism, and they told her it was because of a lack of iodine." " My sister has thyroid disease, and to avoid getting it, I'm taking Kelp tablets." "Can I eat sushi if I take Synthroid?"
Although these questions and comments are relevant, this subject is peppered with "old wives tales" and folklore. In the first part of this discussion, I'd like to focus on the role iodine in relation to the thyroid gland and its function.... a bit of physiology for the beginner. Later, I will discuss how an excess or deficiency of iodine can contribute to diseases of the thyroid gland.
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What is hypothyroidism?Hypothyroidism
is a condition characterized by abnormally low thyroid
hormone production. There are many disorders that result in
hypothyroidism. These disorders may directly or indirectly involve the thyroid
gland. Because thyroid hormone affects growth, development, and many cellular processes, inadequate thyroid hormone has widespread consequences for the body. This article will focus specifically on hypothyroidism in adults.
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% and 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
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, there is a persistent low level of circulating thyroid
hormones.
Next: What causes hypothyroidism? »
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Hypothyroidism
Hypothermia »
What is hypothermia?
The body maintains a relatively stable temperature whereby heat production is
balanced by heat loss. Normally, the core body temperature (when measured
rectally) is 98.6 degrees F or 37 degrees C. When the outside environment gets
too cold or the body's heat production decreases, hypothermia occurs (hypo=less
+ thermia=temperature). Hypothermia is defined as having a core body
temperature less than 95 degrees F or 35 degrees C.
Body temperature is controlled in the part of the brain
called the
hypothalamus, which is responsible for recognizing alterations in the body
temperature and responding appropriately. The body produces heat through the
metabolic processes in
cells that support vital body functions. Most heat is lost at the skin surface
by convection, conduction, radiation, and evaporation. If the environment gets
colder, the body may need to generate more heat by shivering (increasing muscle
activity ...
Read the Hypothermia article »
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