Index to this Health
Guide
Clinical
Features
Neonatal Hypothyroidism
Borderline Hypothyroidism
Treatment
Other Forms of Thyroid Hormone
Duration of Treatment
Treatment of Pituitary or Hypothalamic Hypothyroidism
|
Health
Guides on Thyroid Disease
|
Hypothyroidism or underactive thyroid, occurs when the thyroid gland
fails to produce sufficient amounts of the thyroid hormones T4 and
T3. There are four main causes:
- treatment of Graves' hyperthyroidism
with radioactive iodine or by thyroidectomy;
- end result of Hashimoto's thyroiditis,
an inflammatory process of the thyroid gland; this may occur spontaneously
during the course of Graves' disease;
- birth of a baby, born without
a thyroid gland (congenital hypothyroidism);
- surgical removal of the thyroid
gland as a treatment for thyroid cancer.
H ypothyroidism can also be caused by
disease of either the pituitary gland or the hypothalamus. This is
because normal function of the thyroid gland depends on carefully
regulated secretion of thyroid stimulating hormone (TSH) from the
pituitary gland and thyrotropin releasing hormone (TRH) from the hypothalamus.
Another important, but transient form of hypothyroidism occurs with
postpartum thyroiditis or subacute thyroiditis. |
Clinical
Features
Hypothyroidism affects approximately
2 persons in 100. The signs and symptoms of overt hypothyroidism
are opposite to those in hyperthyroidism since there is a deficiency
of thyroid hormone secretion and all metabolic processes "slow down."
The patient has poor appetite, intolerance to cold, dry, coarse,
skin, brittle hair, tiredness, a croaky, hoarse voice, constipation,
and muscle weakness. Examination may reveal an absence of the thyroid
gland, dry, scaly, cold, pale skin, a thickening of the skin and
underlying tissues (called myxedema), very slow reflexes and a slow
heart rate. The patient can have poor memory retention. The diagnosis
of hypothyroidism is confirmed by finding very low levels of thyroid
hormones (T4 and T3) in the blood.
|
Neonatal
Hypothyroidism
Newborn babies are tested using a
"heelpad blood-spot test." Neonatal hypothyroidism is caused, for
unknown reasons, by an absence of the baby's thyroid gland at birth.
Thyroid hormones are essential for brain development and growth.
New-born infants with hypothyroidism that is not treated, are called
cretins and have severe body and mental defects. These include mental
retardation, poor vision, thick dry skin, protrudent tongue, muscle
weakness, severe lethargy and tiredness. If diagnosed and treated
soon after birth, growth and mental development can proceed relatively
normally.
Much of the research work in making
an early diagnosis of Neonatal Hypothyroidism was carried out in
Canada by Dr. J.H. Dussault at Laval University.
|
Borderline
Hypothyroidism (Compensated Hypothyroidism)
Borderline Hypothyroidism (Compensated
Hypothyroidism) is quite common, and almost impossible to diagnose
clinically. The hallmark is that of an elevated TSH concentration,
with normal or only slightly reduced thyroid hormone levels. There
may be no symptoms, or very vague symptoms, associated with this
condition.
It is important to make the correct
diagnosis because once treatment is started it usually continues
for life as it becomes very difficult to stop treatment to determine
whether the original diagnosis was correct. The measurement of TSH
in the blood helps to define even minor degrees of hypothyroidism.
Top
|
Treatment
Treatment of hypothyroidism is to
take thyroid hormone replacement in the form of a small pill, daily,
for life. This is now given in the form of thyroxine ("eltroxin"
or "synthroid"), a synthetic hormone which has few impurities, very
few side effects and produces almost no allergic reactions. The
dose of thyroxine in adults ranges from 0.1 to 0.2 mg per day. Most
patients require between 0.125 - 0.15 mg but a few require less
and a few require more. There is no need to add T3, since T4 breaks
down to T3, and the dosage is set to provide a normal T3 level.
Once the dose has been established, it is usually stable for life
and patients treated with thyroxine need only have blood tests once
a year. Major stress or illness can sometimes increase the need
for thyroid hormone. Infants and children require smaller doses.
Adult doses are given for teenage patients. Too much thyroxine causes
symptoms of hyperthyroidism whereas symptoms of hypothyroidism persist
with too little. The correct dose is determined from blood tests
of thyroid hormone levels, particularly the total serum triiodothyronine
and TSH tests, and from clinical examination. |
Other
Forms of Thyroid Hormone
There are many other forms of thyroid
hormone but it is very unusual to prescribe any of these. Impure
preparations such as thyroid extract, thyroglobulin, and crude thyroid
preparations contain variable amounts of thyroid hormones. They
produce variable effects and an unpredictable response to treatment.
Triiodothyronine (T3), which is much more potent than thyroxine
is also given on occasion. This drug has a short life span in the
blood and causes irregular stimulation of the heart. Therefore,
T3 ("cytomel") should not be given to patients with heart disease
or to older patients.
|
Duration
of Treatment
Assuming that the diagnosis of hypothyroidism
was correct, treatment for thyroid hormone should always be continued
for life. The cause of thyroid failure is likely to be progressive
and permanent.
Many patients
are given thyroxine for the wrong reasons (such as obesity or tiredness).
Therefore, it is essential that blood tests be carried out and that
thyroid hormone levels are clearly shown to be below the normal
range. Additionally, patients must have symptoms and signs of hypothyroidism.
Hypothyroid patients should not stop
taking thyroid hormone. Thyroid hormone treatment must be continued
even when the patient develops other illnesses, although the dosage
may have to be altered.
|
Treatment
of Pituitary or Hypothalamic Hypothyroidism
The treatment of hypothyroidism caused
by failure of the pituitary or the hypothalamus is also thyroxine.
Pituitary or hypothalamic failure are both very rare compared to
failure of the thyroid gland. In these cases, other hormone deficiencies
may exist which must be identified and treated as well.
Top
|
A
printed version of this Health Guide is available to health care
professionals and the public. For more information call the National
Office or contact your local
chapter.
Production of
the printed version of this Health Guide was made possible through
partial funding assistance from Health Canada. The views expressed
herein are solely those of the authors and do not necessarily represent
the official policy of Health Canada. |
|
