Patient Education - Endocrine Encyclopedia
Endocrine Surgery Encyclopedia
Definition:
Secondary hypothyroidism involves decreased activity of the thyroid caused by failure
of the pituitary gland.
Alternative Names: Pituitary hypothyroidism
Causes, incidence, and risk factors:
The thyroid gland is an important organ of the endocrine system, located in the front
of the neck just below the voicebox. The thyroid secretes the hormones thyroxine
(T4), triiodothyronine (T3), and calcitonin, which control body metabolism and regulate
calcium balance.
The secretion of T3 and T4 by the thyroid is controlled by an endocrine feedback
system involving the pituitary gland and the hypothalamus (structures in the brain).
Lowered levels of these thyroid hormones result in increased levels of pituitary
and hypothalamic hormones. The reverse is also true -- when levels of the thyroid
hormones rise, pituitary and hypothalamic hormones fall back. This helps keep levels
appropriately balanced.
Since the thyroid gland is regulated by the pituitary gland and the hypothalamus,
thyroid disorders may result not only from defects in the thyroid itself but also
from the disruption of the control system in these other organs.
Thyroid disorders caused by overproduction of thyroid hormones are called hyperthyroidism,
and underproduction of these hormones is known as hypothyroidism.
The cause of secondary hypothyroidism is failure of the pituitary gland to secrete
thyroid stimulating hormone (TSH). This is usually caused by a tumor in the region
of the pituitary. Rarely the cause is an infiltration of the pituitary by inflammatory
cells from the immune system or foreign substances (such as iron in hemochromotosis).
Hypothyroidism may cause a variety of symptoms and can affect all body functions.
The body's normal rate of functioning slows, causing mental and physical sluggishness.
Symptoms vary from mild to severe. The most severe form is called myxedema, which
is a medical emergency and can lead to coma and death.
Risk factors for secondary hypothyroidism include being over 50 years old, being
female, and having a history of pituitary or hypothalamic dysfunction.
Symptoms:
Early symptoms:
- Weakness
- Fatigue
- Cold intolerance
- Constipation
- Weight gain
- Depression
- Joint or muscle pain
- Brittle fingernails
- Coarseness, thinning of hair
Late symptoms:
- Slow speech
- Dry, flaky skin
- Thickening of the skin
- Puffy face, hands, and feet
- Decreased hearing
- Thinning of eyebrows
- Hoarseness
- Menstrual disorders
Signs and tests:
A physical exam usually reveals a small thyroid gland. Vital signs (temperature,
pulse, rate of breathing, blood pressure) reveal a slow heart rate, low blood pressure,
and low temperature.
A chest x-ray may reveal an enlarged heart.
Laboratory tests to determine thyroid function include:
- Free T4 test
- Total T3
- Serum TSH -- Results are generally low in secondary hypothyroidism because the
pituitary is damaged. However, normal or even high values may be seen.
Additional laboratory abnormalities may include:
- Increased cholesterol levels
- Increased liver enzymes
- Increased serum prolactin
- Low serum sodium
- Low blood glucose
- A CBC that shows anemia
- Deficiency or excess of other pituitary hormones
- Imaging will include an MRI of the pituitary to look for a tumor.
Treatment:
The purpose of treatment is to replace the deficient thyroid hormone. Levothyroxine
is the most commonly used medication. The lowest dose effective in normalizing thyroid
function is used. Life-long therapy may be necessary. Medication must be continued
even when symptoms subside.
After replacement therapy has begun, report any symptoms of increased thyroid
activity (hyperthyroidism), such as restlessness, rapid weight loss, and sweating.
A high-fiber, low-calorie diet and moderate activity will help relieve constipation
and promote weight loss if weight was gained during the time when thyroid activity
was low.
In individuals with accompanying hypoadrenalism, steroid replacement must be instituted
before thyroid replacement is begun.
In patients who have hypothyroidism caused by a pituitary tumor, surgery may be
required. However, surgery may not cure the hypothyroidism, and thyroid replacement
will still be needed.
Myxedema coma is treated by intravenous (IV) thyroid replacement and steroid therapy.
Supportive therapy of oxygen, assisted ventilation, fluid replacement, and intensive
care nursing may be indicated. Support Groups:
Expectations (prognosis):
With early treatment, return to the normal state is usual. However, relapses will
occur if the medication is not continued. Myxedema coma can result in death. Complications:
Myxedema coma, the most severe form of hypothyroidism, is rare. It may be precipitated
by an infection, illness, exposure to cold, or certain medications.
Symptoms and signs of myxedema coma include:
- Unresponsiveness
- Decreased breathing
- Low blood pressure
- Low blood sugar
- Below-normal temperature
Other complications include:
- Heart disease
- Infertility
- Miscarriage in pregnant women
- Adrenal crisis, if thyroid replacement is begun prior to steroids in hypoadrenal
patients
Calling your health care provider:
Call your health care provider if signs of hypothyroidism are present, or if chest
pain or rapid heartbeat occur.
Call your provider if restlessness, rapid weight loss, sweating, or other symptoms
occur after beginning treatment for this disorder.
Call your provider if headache, visual loss, or breast discharge occur.
Prevention:
This condition may not be preventable. Awareness of risk may allow early diagnosis
and treatment.
Review Date: 8/6/2004
Reviewed By: Aniket R. Sidhaye, M.D., Division of Endocrinology and Metabolism, Johns
Hopkins University School of Medicine, Baltimore, MD. Review provided by VeriMed
Healthcare Network.
 A.D.A.M.,
Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission
(www.urac.org). URAC's accreditation program is an independent audit to verify that A.D.A.M.
follows rigorous standards of quality and accountability. A.D.A.M. is among the first to
achieve this important distinction for online health information and services. Learn more
about A.D.A.M.'s editorial policy, editorial process and privacy policy. A.D.A.M. is also
a founding member of Hi-Ethics and subscribes to the principles of the Health on the Net
Foundation (www.hon.ch).
The information provided herein should not be used during any medical emergency or for
the diagnosis or treatment of any medical condition. A licensed medical professional should
be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for
all medical emergencies. Links to other sites are provided for information only -- they do
not constitute endorsements of those other sites. © 1997- 2007 A.D.A.M., Inc. Any duplication
or distribution of the information contained herein is strictly prohibited.
|
