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Hypothyroidism (underactive thyroid gland)
What is the thyroid?
The normal thyroid is a butterfly-shaped gland that lies at the base of the neck, in front of the
windpipe. It produces hormones that are important for the body's metabolism and organ function.
Thyroid hormone influences conditions such as heart rate, cholesterol level, muscle strength, skin
condition, menstrual cycle and mental state. In children, thyroid hormone is essential for normal
growth and development. Therefore, all newborn babies are now screened for an underactive thyroid
gland with the heel-prick blood test.
How does the thyroid gland function?
The thyroid gland is usually controlled by a hormone released by the pituitary gland (a gland at the
base of your brain) called TSH (thyroid stimulating hormone). The thyroid responds to TSH by
releasing thyroid hormones (known as T4 and T3) which then enter the bloodstream and affect the
metabolism of the body. As part of a feedback mechanism, the pituitary gland monitors the level of
thyroid hormone in the blood and increases or decreases the amount of TSH released, which then
changes the amount of thyroid hormone in the blood.
Hypothyroidism
An underactive thyroid gland (called hypothyroidism) is the most common type of thyroid disorder. It
occurs when the thyroid gland fails to produce enough thyroid hormones (T4 and T3). Hypothyroidism
affects about 2% of women and 0.5% of men and is more common with increasing age. Often there is a
history of hypothyroidism in other family members.
What are the symptoms and signs of hypothyroidism?
Hypothyroidism results in a general slowing of metabolic processes. In adults, symptoms are often
vague and may develop gradually over several years. Most people complain of feeling generally tired
and they may have difficulty tolerating the cold – wearing extra clothes, turning on heaters. The skin
and hair may be dry and there may be puffiness around the eyes. Constipation, hoarseness of the
voice, poor memory, difficulty concentrating, heavy menstrual periods and infertility may occur. Slight
weight gain due to fluid retention is common, but major weight changes are very unlikely to result
from an underactive thyroid.
How is the function of the thyroid tested?
Your doctor can assess your thyroid function with a blood test. This measures the level of TSH in your
blood. The normal range for TSH is usually 0.4 to 4.0 mIU/L. Measurements of T4 (or free T4), and
sometimes T3, may also help your doctor diagnose thyroid problems. The normal range for free T4 is
55 to 160 nmol/L (or 10 to 24 pmol/L) and T3 1.2 to 2.8 nmol/L (free T3 2.5 to 6.0 pmol/L). Some
people make antibodies to their thyroid tissue (similar to the antibodies we make to fight an infection)
and these may also be measured in the blood.
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Issued: 22 July 2014
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How is an underactive thyroid diagnosed?
Hypothyroidism may be difficult to detect because the vague symptoms, and signs are easily confused
with other conditions, such as the natural ageing process, menopause or stress. Diagnosis depends
on measurement of the level of TSH in the blood. A high level of TSH (over 10 mU/L), usually with a
low T4, is seen. If the level of TSH is only slightly elevated (less than 10 mU/L) you are unlikely to have
symptoms (this is called subclinical hypothyroidism) but your doctor may recommend a trial of
thyroxine to see if it improves the way you feel. Thyroid antibodies may also be measured in the blood
sample and indicate a diagnosis of autoimmune hypothyroidism (see below).
Rarely, hypothyroidism is due to failure of the pituitary gland to produce sufficient TSH and in this
case the T4 level is low with a normal or slightly low TSH. Usually other hormones are also affected,
requiring other medication – such as hydrocortisone.
What causes hypothyroidism?
The most common cause of hypothyroidism in adults is autoimmune hypothyroidism (also called
Hashimoto's thyroiditis), where the body's immune system produces antibodies that destroy the
thyroid. As the thyroid gland produces less thyroid hormones, the pituitary gland releases more TSH
to encourage the thyroid to work harder. This increased demand on the thyroid may cause it to
enlarge, resulting in a goitre.
Hypothyroidism may result from previous treatment – thyroid surgery or radioiodine treatment.
Medication such as lithium for mood disorders, amiodarone for the heart or excess iodine in the diet –
kelp or seaweed tablets – can also cause hypothyroidism.
What is the treatment for hypothyroidism?
The goal for treating hypothyroidism is to restore normal blood levels of thyroid hormone by
replacing missing hormone. This is usually relatively simple and involves taking tablets containing a
synthetic hormone called thyroxine (T4) – which is identical to the hormone naturally produced by the
thyroid gland. The other thyroid hormone, called T3, acts more rapidly but has to be given three times
a day. There is no good evidence that treatment with T3 is any better than treatment with T4 and it is
rarely used.
Since hypothyroidism has generally developed gradually, the dose of thyroxine is slowly increased. In
the elderly, or people with heart disease, the dose is increased even more gradually as a rapid increase
in metabolism may stress the heart. A blood test after two to three months is helpful to decide the
correct dosage. After three to six months, when the dose of thyroxine has been stabilised, blood tests
should be repeated every year to check the dose is still correct. Requirements for thyroxine may
change over time depending on age, pregnancy, body weight, other diseases or medications.
The usual maintenance dose of thyroxine for an adult is 50 to 150 µg given as a once-daily dose. The
aim of treatment for hypothyroidism is for the TSH to remain within the lower half of the normal
range, usually 1 to 2 mU/L. If the dose of thyroxine is too high, the TSH will be below the normal
range (suppressed), and if the dose is too low, or tablets are missed, the TSH will be high. Certain
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Hypothyroidism (underactive thyroid gland)
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drugs, such as iron tablets or calcium supplements, can affect the absorption of thyroxine. Taking
these medications well clear of thyroxine will avoid problems.
How long do you need to take thyroxine tablets?
Hypothyroidism is usually permanent and requires lifelong treatment. Occasionally an underactive
thyroid is temporary, particularly if the hypothyroidism occurs after pregnancy.
What happens if you take too much or too little thyroid hormone?
If you are not taking sufficient thyroxine, the symptoms of hypothyroidism – fatigue, feeling the cold,
constipation, and so on – may return. Lack of thyroid hormone for years may cause increased
cholesterol levels and early heart disease. Taking too much thyroxine may lead to symptoms of an
overactive thyroid – heat intolerance, anxiety, irritability, tremor and palpitations. Excess thyroid
hormone for years increases the risk of thin bones (osteoporosis) and heart disease.
Pregnancy
Women with treated hypothyroidism have normal fertility. During pregnancy, the dose of thyroxine
frequently needs to be increased. We recommend that with a positive pregnancy test, the dose of
usual dose of thyroid hormone is doubled for two days each week, and the thyroid function then
tested every one to two months during the pregnancy. After delivery, the dose of thyroid hormone can
be reduced to the pre-pregnancy dose.
What happens if I miss a tablet?
Luckily, thyroxine has a long half-life and stays in your circulation for a reasonable amount of time. If
you forget a tablet, or you are unable to absorb your tablets for a few days because of vomiting or
diarrhoea, you will not come to any harm. Just resume your tablets as soon as possible.
Can people with hypothyroidism live a normal life?
Provided people with hypothyroidism continue to take their tablets regularly, they can lead a normal
active life with no specific physical or occupational restrictions. A yearly blood test should be
performed to check your dosage is correct.
More information
If you wish to obtain more information, recommended web pages include:
National Institutes of Health (www.nlm.nih.gov/medlineplus)
The American Thyroid Association (www.thyroid.org)
Patient.co.uk (www.patient.co.uk/health/hypothyroidism-underactive-thyroid-leaflet)
See also:
Goitre (www.healthinfo.org.nz/81887.htm)
Written by the Department of Endocrinology, Christchurch Hospital.Endorsed by HealthInfo clinical advisers. June 2013.
70690
HealthInfo reference: 70690
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Issued: 22 July 2014
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