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Thyroid Replacement Hormones: Side Effects and Dosage

What are thyroid replacement hormones and how do they work?

Thyroid replacement hormones are medications used to treat hypothyroidism, a
condition in which the production of thyroid hormone in the body is abnormally
low. Thyroid hormones increase cellular metabolism (activity of cells) that is
responsible for growth, development of tissues, maintenance of brain function,
body temperature regulation and several other cellular processes. Low levels of
thyroid hormones in the body can result in many problems given the numerous
activities that they mediate. The thyroid gland, a gland found in the lower neck
is responsible for the production of thyroid hormones. It produces two main
hormones, thyroxine (T4) and triiodothyronine (T3). The hormone responsible for
most of the biological effects in the body is T3. When T4 is released into the
blood by the thyroid gland, most of it is converted to T3 which is responsible
for the cellular metabolic processes. Commercially available thyroid hormones
are either natural or synthetic (man-made). Desiccated thyroid or thyroid
extract (Armor Thyroid, Nature-Throid), a natural thyroid hormone is derived
from beef or pork. Levothyroxine sodium (for example, Synthroid, Levoxyl and
Levothroid), is the synthetic version of thyroxine (T4),

liothyronine sodium (Cytomel, Triostat), is the synthetic version of tT3 and
liotrix (Thyrolar)
is a synthetic thyroid hormone containing a mixture of T4and T3.

What are some examples of thyroid replacement hormones?

The following is a list of the thyroid replacement hormones that are
available in the United States:

  • levothyroxine sodium (Levothroid, Levoxyl, Synthroid, Tirosint,
    Unithroid)
  • liothyronine sodium (Cytomel, Triostat)
  • liotrix
    (Thyrolar)

What are the uses for thyroid replacement hormones?

Thyroid replacement hormones are used to treat hypothyroidism (low production of thyroid hormone) and myxedema, a condition that is caused by prolonged hypothyroidism. Thyroid replacement hormones prevent thyroid hormone release from cancerous thyroid nodules and are used therefore to treat thyroid cancers. They also are used to manage thyrotoxicosis, a condition in which there are high levels of thyroid hormones resulting from over-active thyroid glands and too much thyroid hormone. Thyrotoxicosis may progress to hypothyroidism or cause the growth of goiters necessitating the use of thyroid replacement hormones.

Are there any differences among the different types of thyroid replacement
hormones?

There is conflicting evidence regarding which hormone replacement therapy
should be preferred. The American Association of Clinical Endocrinologists
recommends that clinical hypothyroidism is best treated with synthetic T4 levothyroxine (for example
levothyroxine and sodium [Synthroid, Levoxyl and Levothroid]). There is
variability between the absorption and distribution of generic T4 compared to
brand name preparations. Hence it is recommended that patients remain with
specific brand names during treatment. There is also variability between generic
formulations and brand names of pure T3 (liothyronine [Cytomel, Triostat]), combined T4/T3
formulations (liotrix [Thyrolar])
and thyroid extracts from animal sources (Armour Thyroid, Nature-Throid etc.).
Thyroid extracts from animal sources are no longer available in the United
States. Emerging information shows that combination of T4/T3 therapy may have some
advantages over T4 in cognitive performance and mood but studies are not
conclusive.

What are the side effects of thyroid replacement hormones?

Thyroid replacement hormones usually are well tolerated. Symptoms that occur
during treatment are often due to toxic, elevated levels of thyroid hormones and
resulting symptoms from hyperthyroidism. Symptoms may include
chest pain,
increased heart rate or pulse rate, excessive
sweating, heat intolerance, nervousness, headache, insomnia, diarrhea,
vomiting, weight
loss, and fever.
Some women may experience
irregular
menstrual cycles.

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Which drugs or supplements interact with thyroid replacement hormones?

Thyroid replacement hormones should be used cautiously in people with diabetes since starting or discontinuing therapy may lead to a loss of control of the blood sugar requiring adjustments in doses of insulin or oral antidiabetic drugs (for example, glyburide [Micronase]). The effects of blood thinners such as warfarin (Coumadin) may be increased by thyroid replacement hormones warranting a decrease in the dose of warfarin in addition to monitoring of blood clotting.

Intravenous epinephrine administration in patients with coronary artery disease who are taking thyroid replacement hormones may increase the risk of complications such as difficulty in breathing and possibly heart attacks. The effectiveness of some beta blockers [for example, metoprolol (Lopressor) orpropranolol (Inderal)] may be reduced when a patient is converted from a state of hypothyroidism (under activity) to a normal state (euthyroid state). It also may be necessary to modify the dose of digoxin (Lanoxin) and theophylline (Slo-Bid) when a patient is converted from hypothyroidism (under activity) to a normal state (euthyroid state). There is increased elimination of theophylline in a euthyroid state compared to a state of hypothyroidism.

The effectiveness of thyroid replacement hormones may be decreased when given with drugs such as calcium carbonate, ferrous sulphate, cholestyramine (Questran) and colestipol (Colestid) that binds thyroid replacement hormones and prevent their absorption. This interaction may be reduced by separating the administration of these drugs from thyroid replacement hormones by four hours.

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