Alu elements: Short DNA sequences
that can code for themselves to be copied to other parts of the genome.
They are different from genes. Some Alu elements can serve as responsive
elements and control how genes are expressed. Alu elements are
interesting to geneticists because they can multiply or change their
location throughout evolution and so are different in different species.
Alzheimer's disease: A severe,
degenerative, incurable disease that affects the brain and the ability
to remember, think, and function. See our other Hot Topics: Alzheimer's
Disease: A Family Affair and a Growing Social Problem, and
Antibodies: Substances produced by
the immune system that attack substances the immune system recognizes
as foreign, and not part of the self. Antibodies attach to the foreign
substance and alert the immune system to destroy it.
Antidepressants: Drugs used to treat
clinical depression. They can affect T3 metabolism in the brain.
Autoimmune disease: Disease that
occurs when the body's immune system accidentally recognizes a natural
substance produced by the body as foreign, for unknown reasons, and
attacks that substance. Some autoimmune diseases are insulin-dependent
diabetes mellitus, lupus, thyroid disease, and vitiligo (loss of skin
Cardiovascular fitness: The proper
working of the heart and blood system and its ability to handle
exercise by delivering enough blood, oxygen and fuel to the body.
Cholesterol: A waxy substance that
the body uses to synthesize steroid hormones (sex hormones, coristol,
etc.). When present in excessive quantities in the blood, it increases
the risk of heart disease. Cholesterol can be reduced through exercise
and careful diet if thyroid hormone levels are normal.
Chordates: Animals with spinal
cords. This includes all vertebrates (animals with backbones) and a few
other animals with spinal cords.
Cretinism: Mental retardation and
physical problems caused by hypothyroidism from birth which was not
treated soon enough with thyroid hormone replacements. The U.S. tests
all infants at birth for hypothyroidism to prevent cretinism.
Depressants: Drugs that depress the
brain, slow reflexes, numb sensation, etc. Depressants can affect T3
metabolism by slowing breakdown of T3, which leads to prolonged levels
of T3 beyond appropriate brain function. This may affect mood.
Depression: A condition
characterized by lack of interest in life, numbed emotions or extremely
negative feelings, problems focusing, withdrawal from people and social
life, etc. Depression without apparent cause that does not resolve by
itself is a sign of a serious problem.
Estrogen: Known as the female
hormone, it encourages growth of female reproductive tissue. It can
cancel out the effects of another female hormone, progesterone, and of
thyroid hormone. The more estrogen there is in the body, the more
thyroid hormone is needed for proper function. Many plants (like soy
and corn) have compounds that can mimic the effects of estrogen. Eating
a diet high in these substances can increase the body's requirement for
Fatty acids: Substances created
during the digestion of fat. They travel through the blood and are used
for fuel or are stored by fat cells.
Genes: Stretches of DNA containing
coded instructions to make proteins.
Gene transcription: The process of
reading genes and making proteins according to their instructions;
turning genes on.
Genome: The total set of all genes
and other DNA in the nucleus of each cell.
Goiter: The term for an enlarged
thyroid gland that is trying to make enough thyroid hormone to meet a
hypothyroid body's needs. Goiters can range in size from small enough
to be noticeable only through touch to bigger than a human baby.
Goitrogens: Substances found
primarily in plants that impair the ability of the body to use iodine
to synthesize thyroid hormone. When TH synthesis is affected by
goitrogens, the body may grow a goiter in an effort to make enough
Graves' disease: Autoimmune
hyperthyroidism caused by antibodies continually stimulating the
thyroid to make more TH. Many therapies exist, including total
destruction of the thyroid, followed by thyroid hormone replacement
therapy for the remainder of the patient's life.
Hashimoto's thyroiditis: An
autoimmune thyroid disease that causes goiter, hypothyroidism, and
progressive destruction of the thyroid. It results in unstable thyroid
hormone levels, which can be controlled by thyroid hormone replacement
therapy. There is no known cause or other therapy.
Hormones: Substances released by one
part of the body to influence processes in other parts of the body.
They usually travel in the bloodstream. Insulin, estrogen, growth
hormone, thyroid hormone, and TSH are all hormones.
Hypothyroidism (Hypo=low): The body's
when thyroid hormone levels are insufficent for normal body function.
Symptoms include slowed metabolism, fatigue, mental problems,
hypoglycemia, problems breathing, slow heartbeat, and high cholesterol.
Hyperthyroidism (Hyper=high): The
condition when thyroid hormone levels are excessive. Symptoms include
high metabolism, rapid heart rate, fatigue, and manic-depressive
Iodine: An element (I) found in
nature. It is more common in the ocean than on land, and is essential
for making thyroid hormone. Iodine deficiency leads to hypothyroidism.
A sudden excess in an iodine-deficient person can cause a brief bout of
Ketone bodies: Byproducts
of fatty acid breakdown. When present in excess over a long period of
time, they can be damaging to the body. Ketone body concentrations
increase during episodes of starvation or diabetes mellitus as the body
runs low on glucose (sugar).
Levothyroxine (T4): The relatively
inactive form of thyroid hormone, with four iodine atoms. It is
converted by body tissues to T3 (triiodothyronine) when needed. It
makes up 80% of the thyroid hormone released by the thyroid.
Manic-depression (bipolar disorder): A
characterized by grandiose feelings, accelerated motions and thoughts,
rapid speech, and impulsive decisions. The "manic" phase inevitably
falls back to a normal or very depressive state. Hyperthyroidism can be
mistaken for manic-depression.
Mitochondria: Small organelles
within each cell that have their own genes and act as powerhouses for
the cell, producing ATP for the cell to use for energy and heat.
Neurotransmitters: Chemicals in the
brain that help communicate messages from cell to cell, and are crucial
in thinking, memory, and emotion.
Nuclear receptors: Proteins that are
present in the nucleus and can bind to hormones. When they do so, they
then act as transcription factors and turn on genes.
Proteins: The building blocks of the
body, which are assembled according to genes.
Responsive elements: Short stretches
of DNA, to which transcription factors bind in order to turn genes on
Retinoid x receptors: Nuclear
receptors that are activated by various retinoids. Retinoid X receptors
often are dimerized with other nuclear receptors.
Serotonin: A neurotransmitter that
is essential for a sense of well-being and good mood. Many
anti-depressants work by lifting serotonin levels.
Stimulants: Substances that
stimulate the body's systems, including heart rate, breathing, and
metabolism. They can depress thyroid hormone levels while they are in
Thyroid (from Greek, "shield"): A
butterfly shaped gland behind the larynx that produces thyroid hormone.
It is larger in women than men.
Thyroid hormone (TH): A hormone
containing iodine that affects the body's metabolism. It exists in two
forms, levothyroxine (T4) and triiodothyronine (T3).
Thyroid-stimulating hormone (TSH): The
hormone released by the pituitary gland that stimulates the thyroid to
grow and make more TH.
Thyroid hormone receptors (TRs):
that become activated when they bind to TH and are then able to bind to
responsive elements in the DNA. TRs turn genes on or off when they bind
to the DNA. Mutations in the two genes that code for TRs can cause
symptoms of hypothyroidism.
Triglycerides: Fat by-products. The
presence of triglycerides in the blood is a risk factor for heart
disease and indicates that the body is not metabolizing fat properly.
Triglycerides can be reduced through exercise if thyroid hormone levels
Triiodothyronine (T3): The active form
thyroid hormone. It has three iodine atoms and is eight times more
active than levothyroxine. It is the essential form used in the brain.
Vitamin A: Substances (retinol,
dehydroretinol, retinoic acid) that work like retinol in the body and
are essential to health and development. Vitamin A can be synthesized
in the body from carotenes.