Approach to Thyroid antibody testing
(February 2009)
( Download 
Thyroid Abs Newsletter 2009.pdf)
Thyroid autoantibody testing options:
- Anti-Thyroglobulin (anti-Tg)
- Anti-Thyroid Peroxidase (anti-TPO), previously “anti-microsomal”
- human Thyroid Receptor Antibodies (hTRAb)
These assays detect antibodies which bind to different antigen present in thyroid tissue, the associated conditions overlap, and these assays are often requested together.
Testing for thyroid Stimulating Immunoglobulin (TSI) was quite different. This complicated assay was very specific for Grave’s Disease, but was discontinued in late 2008.
Anti-thyroglobulin and anti-TPO can each be present in patients with Hashimoto’s thyroiditis, Grave’s disease, non-toxic goitre, and normal individuals. Prevalence is higher in women, and increases with age. Presence of such antibodies is therefore not able to differentiate between types of thyroid disease. An absence of both anti-thyroglobulin and anti-thyroid peroxidase antibodies argues strongly against an autoimmune cause of thyroid disease.
Higher titres of anti-TPO do correlate approximately with severity of thyroiditis, and higher titres are more suggestive of impending hypothyroidism than low titres. Subsequent thyroiditis is more likely in those with anti-TPO and/or anti-thyroglobulin antibodies, particularly with high levels. One study showed that, over 20 years, patients without anti-TPO antibodies had a 4% risk of hypothyroidism, whereas those with strongly positive antibodies had up to 53% risk of thyroid failure.
Specialised “Thyroid Receptor Antibody” Assays – TSI
Human thyroid receptor antibody (hTRAb, our preferred term) is a complicated assay in which patient antibodies are assessed for the ability to bind to the TSH receptor.
Thyroid Receptor Antibody assays (hTRAb) may be indicated when unexplained hyperthyroidism is present. These assays are more strongly associated with Graves’ Disease than anti-TPO or anti-Tg, but are not recommended first line testing. These assays are not performed at HAPS and are referred to PaLMS, Sydney.
The historical term “Long-acting Thyroid Stimulator” (LATS) is of unclear meaning, and is not recommended.
Why request thyroid antibodies?
- To define the cause of the thyroid disorder.
- To assess risk of thyroid disorder in those with a personal or family history of polyendocrine disorder
- Prediction of thyroid dysfunction in pregnant women
- With thyroglobulin levels when monitoring for relapse of thyroid malignancy (rare)
Autoantibody testing is unable to determine whether a patient is hyper-, hypo-, or euthyroid.
A positive result – what now?
Hypothyroid patients should receive supplemental thyroxine. Hyperthyroid patients can be treated with anti-thyroid medication (neomercazole or PTU) and/or beta-blockers. Consider an opinion from an endocrinologist if in doubt.
Patients with autoimmune thyroid disease may develop antibodies to other endocrine organs, and vice versa. This is referred to as the thyrogastric cluster, Autoimmune Polyendocrine Syndrome type II or Schmidt’s syndrome. A clinician may consider screening patients at risk for this disorder for B12 deficiency, adrenal insufficiency or diabetes mellitus type 1a. Antibodies towards these organs can be assessed, and if present, may herald organ dysfunction.
Pregnancy and thyroid dysfunction
Up to 2% of pregnancies are associated with thyroid dysfunction, in part due to the effect of placental HCG. Some women can also develop thyrotoxicosis post-partum. Women with anti-TPO antibodies are more likely (50%) to develop post-partum thyroiditis than those without anti-TPO antibodies (2%). This classically presents as thyroiditis in the months post-partum, followed by a hypothyroid phase. The risk is more pronounced if the antibodies persist in the third trimester. 30% of cases develop permanent hypothyroidism.
Thyroid Autoantibody testing at HAPS
Serum (red top) required.
Anti-TPO and anti-Thyroglobulin autoantibodies are recommended first line testing, and requests for “thyroid antibodies” are interpreted as such. These assays are covered by the Medicare Benefits Schedule.
Thyroid Receptor antibody testing (hTRAb), if indicated, is referred to PaLMS, Sydney.
In early 2008 Immunology, HAPS revised the methodology for the anti-TPO and anti-Thyroglobulin antibody testing to ELISA (previously latex agglutination). Although results from these assays are reported in different units, the two methods show excellent correlation.
Costs of testing
Testing for thyroid antibodies will be covered by the Medicare Benefits Schedule (Item 71165), for Medicare Eligible patients.
Frequency of testing
Testing for thyroid antibodies (thyroglobulin and TPO) is performed twice weekly (Tuesdays and Thursdays).
Author
Theo de Malmanche, Staff Specialist, HAPS Immunology. HAPS Immunology would like to thank Dr Huy Tran (Endocrinologist) for his review of this information sheet. The Immunology Department of HAPS can be contacted by telephone (49214018) or facsimile (49214023).