Thyroid hormones are artificially made hormones that make up for a lack of natural hormones produced by the thyroid gland.
The thyroid gland, a butterfly-shaped structure in the lower part of the neck, normally produces a hormone called thyroxine. This hormone controls the rate of metabolism—all the physical and chemical processes that occur in cells to allow growth and maintain body functions. When the thyroid gland does not produce enough thyroxine, body processes slow down. People with underactive thyroid glands feel unusually tired and may gain weight even though they eat less. They may also have trouble staying warm and may have other symptoms, such as dry skin, dry hair, and a puffy face. By making up for the lack of natural thyroxine and bringing the rate of metabolism back to normal, artificially made thyroid hormone improves these symptoms.
Thyroid hormones also may be used to treat goiter
(enlarged thyroid gland) and certain types of thyroid cancer
Thyroid hormones, also called thyroid drugs, are available only with a physician's prescription. They are sold in tablet form. A commonly used thyroid hormone is levothyroxine (Synthroid, Levoxyl, Levothroid).
For adults and teenagers, the usual starting dose of levothyroxine tablets is 0.0125 mg (12.5 micrograms) to 0.05 mg (50 micrograms) per day. The physician who prescribes the medicine may gradually increase the dose over time.
For children, the dose depends on body weight and must be determined by a physician.
Taking thyroid hormones exactly as directed is very important. The physician who prescribes the medicine will figure out exactly how much of the medicine a patient needs. Taking too much or too little can make the thyroid gland overactive or underactive.
This medicine should be taken at the same time every day.
People who take thyroid hormones because their thyroid glands do not produce enough natural hormone may need to take the medicine for the rest of their lives. Seeing a physician regularly while taking this medicine is important. The physician will make sure that the medicine is working and that the dosage is correct.
In patients with certain kinds of heart disease, this medicine may cause chest pains and shortness of breath
. People who have this problem should be careful not to exert themselves too much.
Anyone who is taking thyroid hormones should be sure to tell the health care professional in charge before having any surgical or dental procedures or receiving emergency treatment.
This medicine is safe to take during pregnancy
, but the dosage may need to be changed. Women who are pregnant should check with their physicians to make sure they are taking the proper dosage.
Anyone who has had unusual reactions to thyroid hormones in the past should let his or her physician know before taking the drugs again. The physician should also be told about any allergies
to foods, dyes, preservatives, or other substances.
Before using thyroid hormones, people with any of these medical problems should make sure their physicians are aware of their conditions:
- heart disease
- high blood pressure
- hardening of the arteries
- history of overactive thyroid
- underactive adrenal gland
- underactive pituitary gland
This medicine usually does not cause side effects if the dosage is right. Certain symptoms may be signs that the dose needs to be changed. Check with a physician if any of these symptoms occur:
- changes in appetite
- weight loss
- changes in menstrual period
- tremors of the hands
- leg cramps
- increased sensitivity to heat
- sleep problems
Other side effects are possible. Anyone who has unusual symptoms while taking thyroid hormones should get in touch with his or her physician.
Thyroid hormones may interact with other medicines. This may increase or decrease the effects of the thyroid medicine and may interfere with treatment. Anyone who takes thyroid hormones should not take any other prescription or nonprescription (over-the-counter) medicines without the approval of his or her physician. Among the drugs that may interact with thyroid hormones are:
- Medicine for colds, hay fever, and other allergies
- Medicine for asthma and other breathing problems
- Medicine for diabetes
- Blood thinners
- Diet pills (appetite suppressants)
- Cholesterol-lowering drugs such as cholestyramine (Questran) and colestipol (Colestid).
— A pair of glands located next to the kidneys. The adrenal glands produce hormones that control many body functions.
— A chemical that is produced in one part of the body and then travels through the bloodstream to another part of the body where it has its effect.
— A pea-sized gland at the base of the brain that produces many hormones that affect growth and body functions.
a sudden and dangerous increase of the symptoms of thyrotoxicosis
, seen in patients with severe hyperthyroidism
or in the period immediately following a thyroidectomy
. (However, good postoperative care and the use of radioiodine
ablation techniques have greatly reduced the incidence of this once common postoperative complication.) Called also thyroid
or thyrotoxic storm
Thyroid crisis is a serious event that can be fatal if not brought under control. All of the body processes are accelerated to dangerously high levels. The pulse may rise to 200 beats per minute, and there is concurrent rise in the respiratory rate. The temperature control center loses control, bringing about a rapid and steady increase in body temperature. Pulmonary edema and congestive heart failure can also occur.Treatment
is aimed at correction of the hyperthyroidism
, control of the symptoms, and prevention of further crisis by treating the underlying cause. Medications are employed to block synthesis of thyroid hormones, block their release, and inhibit conversion of triiodothyronine
. plasma exchange
, or hemodialysis
may be necessary to remove the hormones from the circulation.
Supportive care includes administration of oxygen and measures to control hyperthermia
, such as the use of ice packs or a hyperthermia blanket. Intravenous hydration is important to prevent shock
. The use of glucocorticoids
is associated with improved survival rates. propranolol
, and guanethidine
are often used, as well as other medications that treat symptoms.
the largest of the endocrine glands
, consisting of two lateral lobes connected by an isthmus; a third pyramidal lobe sometimes extends up from the isthmus. The thyroid gland is located in the front and sides of the neck just below the thyroid cartilage and produces hormones that are vital in maintaining normal growth and metabolism (see thyroid hormones
). It also serves as a storehouse for iodine.
Diagnostic tests for thyroid disorders include radioimmunoassay for T3
, and thyroid-stimulating hormone (TSH), free thyroxine serum concentration, and free thyroxine index (FTI). These and other thyroid function tests can be distorted by preparations and foods containing iodine, and by oral contraceptives, phenytoin (Dilantin), and several other drugs. The thyroid scan
is useful in detecting nodules and active thyroid tissue and, combined with radioactive iodine uptake, measures the ability of the thyroid gland to take in ingested iodine.
Persons who received radiation to the head and neck as children are at higher than normal risk for development of thyroid abnormalities. Of these disorders about one-third are carcinomas of the thyroid. Other problems related to radiation early in life include adenomas and other malignant and benign tumors, hypo- and hyperthyroidism, and thyroiditis. The American Thyroid Association suggests periodic laboratory testing and physical assessment of persons at high risk in order to detect these abnormalities when they are more amenable to treatment.
iodothyronines secreted by the thyroid gland
, principally thyroxine
(tetraiodothyronine or T4
) and triiodothyronine
). The serum level of T4
is normally 45 to 50 times the level of T3
. However, T3
is several times more active than T4
, and most T3
is produced by metabolism of T4
in peripheral tissues. The pharmaceutical names for T4
, respectively. Thyroid hormones influence many metabolic processes. They stimulate the cellular production of heat; stimulate protein synthesis; regulate many aspects of carbohydrate metabolism; stimulate lipid synthesis, mobilization, and degradation; stimulate the synthesis of coenzymes from vitamins; and may affect the response of tissues to epinephrine and norepinephrine.
Secretion of thyroid hormones is regulated by the hypothalamus-pituitary-thyroid control system. Internal environmental conditions, such as low thyroid hormone and norepinephrine serum levels, or external factors, such as cold and stress, activate the hypothalamus, which secretes thyrotropin-releasing hormone (TRH). This hormone acts on the pituitary gland and brings about the release of thyroid-stimulating hormone (TSH). The TSH then stimulates the release of thyroid hormones such as T3
from the thyroid gland. When sufficient levels of serum thyroxine and other thyroid hormones have been reached, there is negative feedback to the hypothalamus and TRH is no longer secreted. See also hypothyroidism
1. resembling a shield.
2. the thyroid gland (see below) secreting thyroid hormones (see below).
3. a pharmaceutical preparation of cleaned, dried, powdered thyroid gland, obtained from those domesticated animals used for food by humans.
an additional thyroid located anywhere from the larynx to diaphragm, e.g. intrapericordial aorta; may be sufficient to supply the patient's need of thyroid hormone. Most common in dogs.
the shield-shaped cartilage of the larynx.
primordium of the thyroid gland; appears as an outgrowth of the foregut between the first two pharyngeal pouches. This tube of epithelial cells grows ventrally into mesenchyme; the tube becomes the thyroglossal duct, the tip divides into two lobes, the thyroid glands.
migration of thyroid diverticulum tissue to aberrant sites occurs, e.g. thyroid tissue in the thymus. These cause no apparent abnormality.
a pharmaceutical substance derived from thyroid glands, used in the treatment of hypothyroidism. See thyroid (3) (above).
thyroid function tests
used to assess the functional capacity of the thyroid glands; most commonly employed in dogs and cats. Include plasma T4, plasma T3, free T4 radioiodine uptake, and TSH response tests.
the largest of the endocrine
glands, situated in the neck caudal to the larynx. It produces hormones (see below) that are vital in maintaining normal growth and metabolism. It also serves as a storehouse for iodine.
Microscopic appearance of the thyroid gland. By permission from Guyton R, Hall JE, Textbook of Medical Physiology, Saunders, 2000
iodothyronines secreted by the thyroid gland, principally thyroxine (tetraiodothyronine, T4
) and tri-iodothyronine
); derived from iodination of tyrosyl residues in thyroglobulin. The pharmaceutical names for T4
are levothyroxine and liothyronine, respectively. Regulate basal metabolic rate.
thyroid parafollicular cell thyroid radioiodine uptake
used as a thyroid function test but superseded these days by estimates of T4 (thyroxine).
thyroid-stimulating hormone (TSH) thyroid tumor
mostly follicular adenomas in old-aged dogs, cats and horses; papillary adenomas are rare.