How is it used?
Thyroid antibody testing is primarily ordered to determine whether a patient with thyroid disease has an underlying autoimmune disorder. These tests may be ordered to help investigate the cause of an enlarged thyroid gland (goitre) and/or performed as a follow-up when other thyroid test results (such as FT3, FT4, and/or TSH) show signs of thyroid dysfunction.
One or more thyroid antibody tests may also be ordered if a person with a known non-thyroid-related autoimmune condition, such as systemic lupus erythematosus, rheumatoid arthritis, or pernicious anaemia, develops symptoms that suggest thyroid involvement. This involvement may occur at any time during the course of the other condition(s).
Anti-thyroglobulin is requested with thyroglobulin in patients being monitored following surgical removal of the thyroid for thyroid cancer (see below).
When is it requested?
As mentioned above, thyroid antibodies will be requested if you have clinical features of thyroid disease.
- Common features of hypothyroidism (under-active thyroid) include weight gain, fatigue, goitre (enlarged thyroid gland), dry skin, hair loss, intolerance to cold, and constipation.
- Common features of hyperthyroidism (over-active thyroid) include sweating, rapid heart rate, anxiety, tremors, fatigue, difficulty sleeping, sudden weight loss, and protruding eyes.
|Thyroid peroxidase antibody
||Hashimoto’s thyroiditis; Graves’ disease
||When a patient has symptoms suggesting thyroid disease; when a doctor is considering starting a patient on a drug therapy that may cause thyroid dysfunction when TPOAb are present, (e.g. lithium, amiodarone, interferon alpha, or interleukin-2).
||Has been associated with reproductive difficulties, such as miscarriage, , premature delivery, and in-vitro fertilisation failure
||Hashimoto’s thyroiditis; in some patients being monitored for thyroid cancer
||Whenever a thyroglobulin test is performed to see if the antibody is present and likely to be interfering with the test results; at regular intervals after thyroid cancer treatment; when symptoms of hypothyroidism are present
||Not as sensitive as TPO antibodies in diagnosing autoimmune thyoid disease
|Thyroid stimulating hormone receptor antibody
||Graves’ disease; Hashimoto's thyroiditis
||When a patient has symptoms of hyperthyroidism; to monitor effectiveness of anti-thyroid therapy
||Most TSH receptor antibodies cause hyperthyroidism, but a small group of patients with TSH receptor antibodies will develop hypothyroidism
If a pregnant woman has a known autoimmune thyroid disease (such as Hashimoto’s thyroiditis or Graves’ disease) or has another autoimmune disorder and thyroid involvement is suspected, then one or more of the thyroid antibodies may be ordered early in the pregnancy and then again near the end. These tests are used to help the doctor determine whether the baby may be at risk of thyroid dysfunction since thyroid antibodies can cross the placenta and cause hypothyroidism or hyperthyroidism in the fetus or newborn.
Thyroid antibody testing may also be ordered when a patient with another autoimmune disorder has symptoms of thyroid dysfunction and/or when she has reproductive difficulties that the doctor suspects may be associated with autoantibodies.
What does the test result mean?
Negative test results means that autoantibodies are not present in the blood at the time of testing and may indicate that symptoms are due to a cause other than autoimmune thyroid disease. However a small group of patients with autoimmune thyroid disease do not have detectable thyroid autoantibodies.
Mild to moderately elevated levels of thyroid antibodies may be found in a variety of thyroid and autoimmune disorders, such as thyroid cancer, Type 1 diabetes, rheumatoid arthritis, pernicious anaemia, and autoimmune collagen vascular diseases. Significantly increased concentrations most frequently indicate thyroid autoimmune diseases such as Hashimoto’s thyroiditis and Graves’ disease.
In general, the presence of thyroid antibodies suggests that there is autoimmune thyroid involvement and the higher the level, the more likely this is the case. Rising levels may be more significant than stable levels as they indicate an increase in autoimmune activity. All of these antibodies, if present in a pregnant woman, can increase the risk of hypothyroidism or hyperthyroidism in the developing baby or newborn.
If TGAb are present when monitoring thyroglobulin levels in a patient who has been treated for thyroid cancer, the antibodies may interfere with the thyroglobulin test result. This may mean that the doctor will not be able to use thyroglobulin as a tumour marker for thyroid cancer. If TGAb is being used as a monitoring tool and has stayed high or dropped low initially but is increasing over time, then it is possible that the treatment has not been effective and the condition is continuing or recurring. If levels are falling and/or have fallen to low or undetectable levels, then it is more likely that the thyroid cancer is being treated effectively. Nonetheless, thyroglobulin results must be interpreted with extreme caution in patients with anti-TG antibodies, and it is likely that another test such as a thyroid ultrasound will be required to assess for any evidence of cancer recurrence.
A certain percentage of patients who are healthy may be positive for one or more thyroid antibodies. The prevalence tends to be higher in women and tends to increase with age. If a person with no apparent thyroid dysfunction has a measurable thyroid antibody, their doctor will track his/her health over time. While most may never experience thyroid dysfunction, a few may develop it in the future.
Is there anything else I should know?
The and of thyroid antibody testing is improving but is still not as good as doctors would like it to be. All of the thyroid antibody tests have changed over time. This is part of the reason that these tests have gone by so many names. There are a lot of different methodologies for thyroid antibodies and each of them is slightly different in what it is measuring and will have a different reference interval.
If you are having serial testing done (for monitoring purposes), it is important to have testing done by the same laboratory with the same methodology.
Adding to the complexity of thyroid antibody testing is the fact that the antibodies are not homogenous. This means that the antibodies produced by one person will be slightly different to those produced by another. The thyroid antibody tests pick up varying amounts of the antibody population in the blood, affecting their sensitivity.