Is a TSI Level Less Than 89 Considered Normal?

A TSI level below 89 falls within the normal range on every major assay platform currently used in clinical laboratories. Most TSI tests set their positive cutoff well above 89, so a result in that range is interpreted as negative for thyroid-stimulating immunoglobulins. The tricky part is that “89” means different things depending on which test your lab ran, because TSI results are reported in different units with different reference ranges. Understanding which assay produced your number is the only way to know exactly where you stand.

Why the Cutoff Depends on the Assay

TSI is not a single standardized test the way, say, a basic blood glucose measurement is. Several commercial platforms measure these antibodies, and each reports results in its own units with its own normal range. The most common reporting formats you will encounter on a lab slip are a percentage of baseline activity, an index value, or international units per liter (IU/L). A result of “89” on one platform is not equivalent to “89” on another.

On the older-style bioassay that reports results as a specimen-to-reference ratio expressed as a percentage, the positive cutoff is typically 140%. A value of 89% on that scale is solidly negative. On the newer Siemens Immulite platform, results come back in IU/L, and the manufacturer’s cutoff for a positive result is 0.55 IU/L. One study found an optimal diagnostic cutoff of about the same value when evaluating patients with Graves’ disease against healthy controls.¹ If your lab reported a number like “89” in IU/L, that would actually be extremely high, far above any normal cutoff. But most labs using that platform report results well under 1.0 for healthy people, so a raw number of 89 almost certainly came from a percentage-based assay where 140% is the threshold for concern.

The practical takeaway: check the units printed on your lab report and the reference range your specific laboratory lists. Labs always print a reference range next to the result. If your result is below that range’s upper limit, it is considered negative regardless of the absolute number.

What TSI Actually Measures

Thyroid-stimulating immunoglobulins are antibodies your immune system produces that mimic the hormone your pituitary gland normally uses to tell your thyroid to make more thyroid hormone. When these antibodies latch onto receptors on thyroid cells, the thyroid revs up and overproduces hormones, leading to hyperthyroidism. This mechanism is the hallmark of Graves’ disease, the most common cause of an overactive thyroid.

A TSI test specifically looks for antibodies that stimulate the thyroid receptor, as opposed to antibodies that simply bind to it or block it. That distinction matters clinically, because not all antibodies directed at the thyroid receptor cause the same effect. Some block the receptor and can actually lead to an underactive thyroid instead. The TSI test is designed to detect the stimulating variety.

Your doctor typically orders a TSI test when blood work shows signs of hyperthyroidism and the question is why. The test helps distinguish Graves’ disease from other causes of an overactive thyroid, like a hot nodule or thyroiditis. In one study, every patient with active Graves’ disease had detectable TSI, while patients with toxic adenoma, cold nodules, and non-toxic goiter did not.² That kind of clean separation makes TSI a powerful diagnostic tool.

How Accurate Modern TSI Tests Are

The diagnostic performance of current TSI assays is genuinely impressive. A systematic review and meta-analysis that pooled data from multiple studies found the TSI assay on the Siemens Immulite platform had a sensitivity around 97% and specificity around 96% for diagnosing Graves’ disease.³ In plain terms, the test catches nearly all true cases and rarely flags someone who does not have the disease.

A head-to-head comparison of three different assay formats in patients with Graves’ eye disease found similar results. A newer rapid bioassay had the highest sensitivity at about 97%, while a bridge-based binding assay had perfect specificity, meaning zero false positives in that particular study.⁴ These numbers are high enough that a negative TSI result provides strong reassurance against a Graves’ disease diagnosis, though no test is flawless.

One related test you may see on lab reports is TRAb (thyrotropin receptor antibodies), which detects any antibody that binds to the thyroid receptor, whether it stimulates or blocks. TSI and TRAb overlap considerably in what they catch, but TSI tends to be slightly more sensitive while TRAb can be slightly more specific depending on the assay and clinical scenario.¹ Ordering both tests simultaneously is common but often unnecessary, and one analysis found that paired orders inflated potentially unnecessary testing costs by anywhere from about 30% to over 300%.⁵

When a Normal TSI Result Does Not End the Conversation

A normal TSI value does make Graves’ disease much less likely, but it does not rule out every thyroid problem. Other causes of hyperthyroidism, like toxic multinodular goiter or subacute thyroiditis, would be expected to show a normal TSI anyway. If your thyroid hormones are still elevated despite a negative TSI, your doctor will look for these alternatives using imaging or a radioactive iodine uptake scan rather than repeating the antibody test.

There is also a scenario where TSI can be misleadingly abnormal. High-dose biotin supplements, which some people take for hair or nail health, can interfere with thyroid assays that use biotin-streptavidin chemistry. One case series documented patients on 20 to 30 milligrams of biotin daily whose thyroid function tests shifted enough to suggest Graves’ disease when they did not actually have it.⁶ If you are taking biotin, mention it to your doctor before any thyroid blood draw. Most labs now recommend stopping biotin for at least 48 to 72 hours beforehand.

Another wrinkle involves amiodarone, a heart rhythm medication that can cause thyroid problems. Some patients on amiodarone develop thyrotoxicosis and even test positive for thyroid receptor antibodies, yet not all of them have true Graves’ disease. Researchers identified two distinct groups among amiodarone-induced thyrotoxicosis patients who tested TRAb-positive: one group had genuinely stimulating antibodies that behaved like classic Graves’ disease, while the other had antibodies that bound the receptor without actually stimulating it.⁷ The functional TSI bioassay helped sort out which was which, because only the first group showed true stimulatory activity. This is a niche situation, but it illustrates why the specific type of antibody test matters.

TSI as a Tool for Monitoring Graves’ Disease Treatment

Beyond diagnosis, TSI levels have a second life as a monitoring tool for people already being treated for Graves’ disease. The standard first-line treatment involves antithyroid drugs, typically taken for 12 to 18 months before the doctor considers stopping them. The big question at that point is whether the disease will come back.

TSI levels tend to fall during treatment as the autoimmune process quiets down. If they remain elevated at the time the medication is withdrawn, the risk of relapse is substantially higher. One study found that patients whose TSI was still positive when they stopped their antithyroid medication had dramatically higher odds of relapsing compared to those whose TSI had normalized.⁸

Research has tried to pin down a specific TSI cutoff that best predicts relapse. One study using the Immulite platform identified a cutoff of about 1.31 IU/L: patients whose TSI was below that number before stopping medication had roughly an 87% chance of staying in remission for at least a year.⁹ Patients above that threshold relapsed at significantly higher rates. The test is not perfect as a crystal ball, with sensitivity and specificity both in the 60 to 80% range for this particular prediction, but a low TSI at the end of treatment is genuinely reassuring.

A separate study using a different bioassay that reports results as a percentage found a cutoff of about 66.5% for predicting relapse, though it performed modestly with sensitivity and specificity both around 57 to 61%.¹⁰ The inconsistency between studies reflects the reality that predicting autoimmune disease relapse is inherently tricky and no single blood test does it perfectly. Still, the trend in the data is clear: the lower your TSI goes during treatment, the better your chances of staying well after stopping medication.

TSI and Thyroid Eye Disease

Graves’ disease sometimes affects the eyes, causing swelling, bulging, redness, and double vision in a condition called thyroid eye disease. TSI levels are not just relevant to the thyroid itself but also correlate with how active and severe this eye involvement is. Patients with more active eye disease tend to have higher TSI levels, and TSI values differ significantly between patients with mild versus severe eye involvement.¹¹

This connection has become clinically actionable. Teprotumumab, a medication approved for thyroid eye disease, targets a receptor involved in the disease process, and TSI levels are used to track treatment response. For people who have already been diagnosed with Graves’ disease and have a normal TSI, active thyroid eye disease is less likely, though not impossible.

In children, the connection between TSI and eye involvement has also been documented. A study of 49 pediatric Graves’ disease patients found that roughly 69% of children with positive TSI at diagnosis went on to develop Graves’ eye disease, compared to only 24% of children whose initial TSI was normal or borderline.¹² So even in younger patients, the initial TSI level carries prognostic weight beyond just confirming a thyroid diagnosis.

TSI in Pregnancy

TSI takes on a different significance in pregnant women with Graves’ disease. These antibodies are immunoglobulins, which means they cross the placenta and can stimulate the baby’s thyroid. This can cause fetal or neonatal hyperthyroidism, a serious condition that requires close monitoring and sometimes treatment immediately after birth.

The maternal TSI cutoff for diagnosing Graves’ disease in pregnancy studies is typically set at 1.3 on the specimen-to-reference ratio scale, representing 133% of basal activity. But the threshold that matters for the baby is higher. One study found that neonatal thyroid dysfunction did not occur when the mother’s TSI was below about 2.5 to 2.6, regardless of whether the mother was taking antithyroid medication or thyroid hormone replacement.¹³ Mothers with much higher levels, averaging 12 to 17 in that study, had babies who developed thyroid problems.

This means a normal TSI in a pregnant woman with a history of Graves’ disease is particularly good news. It suggests the autoimmune process has quieted enough that the antibodies are unlikely to affect the baby. Conversely, a TSI that remains very elevated in the third trimester signals the need for neonatal thyroid screening right after delivery. Many endocrinologists and obstetricians check TSI levels in the third trimester specifically for this reason, even if the mother’s own thyroid function is well controlled on medication.

What a “Borderline” TSI Means

Some people get results that are technically below the positive cutoff but close to it, and naturally wonder whether that matters. A TSI of 89% on a scale where 140% is positive is not really borderline; it is firmly negative. But a result of, say, 130% on the same scale might prompt different conversations.

Labs sometimes report a gray zone or “indeterminate” range. If your result lands there, the usual approach is to repeat the test after a few weeks or to add complementary testing like TRAb or a radioactive iodine uptake scan. A single borderline result does not mean you have Graves’ disease. Antibody levels fluctuate, and a borderline result could reflect a very early stage of autoimmune thyroid activity, a lab quirk, or simply where your immune system happens to sit without any clinical consequence.

If you have symptoms of hyperthyroidism, such as rapid heartbeat, weight loss, tremor, heat intolerance, or anxiety, but your TSI is negative, your doctor will not dismiss those symptoms. They will investigate other causes. A negative TSI shifts the diagnostic path away from Graves’ disease but does not mean nothing is wrong.

Differences Between Lab Platforms

The landscape of TSI testing has evolved considerably over the past decade. Older bioassays measured the ability of a patient’s serum to stimulate thyroid cells in a culture dish, reporting results as a percentage of baseline stimulation. These were labor-intensive and available only at reference laboratories. Newer immunoassays, including the widely used Siemens Immulite platform, measure TSI through automated binding assays that are faster, more standardized, and available at more labs.

The shift from percentage-based bioassays to IU/L-based immunoassays has created some confusion for patients tracking their results over time. If you were monitored with one assay and your lab switched to another, the raw numbers are not directly comparable. A drop from 200% to 89% on the old bioassay and then a result of 0.4 IU/L on the new platform all point in the same direction (toward normal), but you cannot plot them on the same graph as though they are the same measurement.

If you are being monitored for Graves’ disease treatment and your lab changes platforms, ask your endocrinologist whether the new baseline needs to be re-established. The trend over time on a single consistent assay is far more informative than any individual number.