Thyroid Stimulating Immunoglobulin, Serum (TSI)

Test Code: TSI

Overview
Ordering
Specimen
Performing
Clinical/Interpretive
Contacts
Coding

Synonyms/Keywords

Ref Lab Code: 8634, Autoimmune Thyroid Stimulator, Graves Disease, LATS (Long-Acting Thyroid Stimulator),
Long-Acting Thyroid Stimulator (LATS), Thyroid Receptor Antibody, Thyroid Stimulating Antibody,
TSH Receptor Binding Inhibitory Immunoglobulin, TSI (Thyroid-Stimulating Immunoglobulin)

Useful For

Second-order testing for autoimmune thyroid disease, including:

-Differential diagnosis of etiology of thyrotoxicosis in patients with ambiguous clinical signs and/or contraindicated (eg, pregnant or breast-feeding) or indeterminate thyroid radioisotope scans

-Diagnosis of clinically suspected Graves disease (eg, extrathyroidal manifestations of Graves disease: endocrine exophthalmos, pretibial myxedema, thyroid acropachy) but normal thyroid function tests

-Determining the risk of neonatal thyrotoxicosis in a fetus of a pregnant female with active or past Graves disease

-Differential diagnosis of gestational thyrotoxicosis versus first trimester manifestation or recurrence of Graves disease

-Assessing the risk of Graves disease relapse after anti-thyroid drug treatment

Specimen Requirements

Specimen Type	Preferred Container/Tube	Acceptable Container/Tube	Specimen Volume	Specimen Minimum Volume (allows for 1 repeat)	Pediatric Minimum Volume (no repeat)
Serum	Red Top Tube (RTT)	Serum Separator Tube (SST)	0.5 mL	0.1 mL

Specimen Stability Information

Specimen Type	Temperature	Tim e
Serum	Frozen (preferred)	60 days
	Room Temperature	24 hours
	Refrigerated	7 days

Specimen Type

Temperature

Tim

Serum

Frozen (preferred)

60 days

Room Temperature

24 hours

Refrigerated

7 days

Rejection Criteria

Hemolysis	Mild OK; Gross reject
Lipemia	Mild OK; Gross OK
Icterus	Mild OK; Gross OK

Performing Laboratory Information

Performing Location	Day(s) Test Performed	Analytical Time	Methodology/Instrumentation
Mayo Medical Laboratories	Monday through Friday	2 days	Recombinant Bioassay

Reference Lab

Mayo Clinic Laboratories

Test Information

Autoimmune thyroid disease is characterized by the presence of autoantibodies against various thyroid components, namely the thyrotropin receptor (thyroid-stimulating hormone receptor [TSHR]), thyroid-peroxidase (TPO), and thyroglobulin (Tg), as well as an inflammatory cellular infiltrate of variable severity within the gland. Among the autoantibodies found in autoimmune thyroid disease, TSHR autoantibodies are most closely associated with disease pathogenesis. All forms of autoimmune thyrotoxicosis (Graves disease, Hashitoxicosis, neonatal thyrotoxicosis) are caused by the production of TSHR-stimulating autoantibodies. The role of the TPO and Tg autoantibodies in either autoimmune thyrotoxicosis or autoimmune hypothyroidism is less well established; they may merely represent epiphenomena. Detectable concentrations of anti-TPO antibodies are observed in most patients with autoimmune thyroid disease (eg, Hashimoto's thyroiditis, idiopathic myxedema, and Graves disease).

Autoantibodies that bind and transactivate the TSHR lead to stimulation of the thyroid gland independent of the normal feedback-regulated thyroid-stimulating hormone (TSH) stimulation. These TSHR autoantibodies also are known as long-acting-thyroid-stimulator or thyroid-stimulating immunoglobulins (TSI). Some patients with Graves disease also have TSHR-blocking antibodies, which do not transactivate the TSHR. The balance between TSI and TSHR-blocking antibodies, as well as their individual titers, are felt to be determinants of Graves disease severity. At least 20% of patients with autoimmune hypothyroidism also have evidence either of TSHR-blocking antibodies or, less commonly, TSI.

TSHR autoantibodies may be found before autoimmune thyrotoxicosis becomes biochemically or clinically manifest. Since none of the treatments for Graves disease are aimed at the underlying disease process, but rather ablate thyroid tissue or block thyroid hormone synthesis, TSI may persist after apparent cure.

TSI are IgG antibodies and can therefore cross the placental barrier, causing neonatal thyrotoxicosis.

Reference Range Information

< or =1.3 TSI index

Reference values apply to all ages.

Interpretation

The sensitivity and specificity of an elevated thyroid-stimulating immunoglobulins (TSI) index for Graves disease diagnosis depends on whether patients have clinically active, untreated disease or disease treated with antithyroid drugs. Using a TSI index of 1.3 as the cutoff level in newly diagnosed, untreated patients, the sensitivity and specificity are >90%. For a higher cutoff of 1.8, specificity approaches 100%, but sensitivity decreases somewhat. In patients with inactive or treated Graves disease the specificity is similar, while sensitivity is lower, ranging from 50% to 80%.

Significant neonatal thyrotoxicosis is likely if a pregnant woman with a history of Graves disease has a TSI index of >3.9 during the last trimester, regardless of her remission status. Lesser elevations are only occasionally associated with neonatal thyrotoxicosis. This is particularly relevant for women who have previously undergone thyroid-ablative therapy or are on active antithyroid drug treatment and, therefore, no longer display biochemical or clinical evidence of thyrotoxicosis.

Gestational thyrotoxicosis, which is believed to be due to a combination of human chorionic gonadotropin cross-reactivity on the thyroid-stimulating hormone receptor (TSHR) and transient changes in thyroid hormone protein binding, is not associated with an elevated TSI index. Finding an elevated TSI index in this setting suggests underlying Graves disease.

An elevated TSI index at the conclusion of a course of anti-thyroid drug treatment is highly predictive of relapse of Graves disease. However, the converse, a normal TSI index, is not predictive of prolonged remission.

In patients with thyroid function tests that fluctuate between hypo- and hyperthyroidism or vice versa, a clearly elevated TSHR-antibody level (>25%) and a simultaneous TSI index that is normal or only minimally elevated (1.3-1.8) suggest a diagnosis of possible Hashitoxicosis.

Outreach CPTs

CPT	Modifier (if needed)	Quantity	Description	Comments
84445