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Hashimoto’s thyroiditis – the autoimmune disease that causes most cases of hypothyroidism in the U.S. – is usually diagnosed by blood tests that show elevated thyroid peroxidase (TPO) antibodies. The purpose of the TPO antibody test is to determine whether the immune system is producing antibodies that attack the thyroid gland itself. Over time, these antibodies can slow down and even destroy the thyroid’s ability to produce thyroid hormone, eventually causing overt hypothyroidism. But can you have Hashimoto’s thyroiditis and have normal antibodies?
For patients with Hashimoto’s, around 90% will have elevated TPO antibodies, consistent with autoimmune thyroid disease. The remaining 10% of patients with Hashimoto’s present a bit of a challenge. They may have a family history of autoimmune disease and symptoms that indicate Hashimoto’s and hypothyroidism, but their TPO antibodies fall in the normal range.
This condition is called Serum Negative Chronic Autoimmune Thyroiditis – or SN-CAT – and is considered a milder version of Hashimoto’s. Patients with SN-CAT can still develop classic Hashimoto’s and are at higher risk of progressing to other autoimmune conditions, such as seronegative rheumatoid arthritis.
SN-CAT patients often have smaller thyroid glands – compared to Hashimoto’s patients with elevated antibodies. These patients are also less likely to have an enlarged thyroid, known as goiter.
According to the American Thyroid Association, the diagnosis of SN-CAT follows several steps.
An ultrasound can be performed and may show thyroid gland inflammation and characteristics of a hypoechoic pattern that indicate Hashimoto’s thyroiditis. On ultrasound scans, hypoechoic structures are seen inside the thyroid gland, appearing darker than the surrounding tissue. Other signs include a heterogeneous echotexture – small areas of increased and decreased density.
While the prevalence of SN-CAT is still low, the increased use of ultrasound has improved SN-CAT detection in recent years. Most doctors, however, don’t order thyroid ultrasound testing unless they see an enlargement of the thyroid gland, which is why SN-CAT often goes undiagnosed.
2. High-normal TSH levels.
SN-CAT is also suspected when two blood tests – done within two to six months of each other – show Thyroid Stimulating Hormone (TSH) levels greater than 4.0 mU/ml.
Interestingly, a study of 55 patients with SN-CAT and 110 patients with classic Hashimoto’s, demonstrates how complicated it is to identify SN-CAT. On average, the patients with SN-CAT had lower TSH levels and higher free T4 levels compared to patients with Hashimoto’s and elevated TPO antibodies. Free T3 levels had similarities for both groups of patients.
3. Normal TPO Tests.
Two separate TPO antibody tests must come back within the reference range for a diagnosis of SN-CAT. (In most cases of typical Hashimoto’s, TPO antibodies are elevated at levels above 34 IU/mL (international units per millimeter).
In many cases, biopsies can definitively diagnose all forms of Hashimoto’s, including SN-CAT. But biopsies are never done solely to detect Hashimoto’s disease. However, when a thyroid biopsy is performed for other reasons – such as to evaluate nodules for thyroid cancer – it may reveal SN-CAT or Hashimoto’s. Recent studies have shown that fine needle aspiration (FNA) thyroid biopsies reveal an unexpectedly high prevalence of Hashimoto’s that was undetected by TPO blood testing.
In symptomatic SN-CAT, the treatment is thyroid hormone replacement medication. In a study of 581 patients with primary autoimmune hypothyroidism, 20.8% had SN-CAT. As the subjects were studied and results evaluated over the years, their age, TSH level, and TPO antibody status correlated to their daily dose of thyroid medication. The study concluded that for those patients with SN-CAT, their thyroid condition was milder than with classic Hashimoto’s thyroiditis, and a lower dose of medication was generally needed.
It is worth noting that lifestyle changes and healthy routines such as an anti-inflammatory diet, improved sleep patterns, stress-relieving practices, and physical activity can affect the immune system’s overactivation and the progress and severity of the condition itself.
There are still many unanswered questions about SN-CAT. What is the risk that an SN-CAT patient will develop full-blown Hashimoto’s? Would pregnancy affect thyroid function and pregnancy outcomes for a woman with SN-CAT? More research is definitely needed to understand this phenomenon.
If you have been diagnosed with – or suspect that you have – SN-CAT, frequent testing is necessary to monitor your thyroid function and the possible progression to overt hypothyroidism and elevated TPO antibodies. Paloma’s affordable at-home thyroid test kit makes regularly testing your thyroid easy and convenient. The kit measures your TSH, Free T4, Free T3, and TPO antibodies, and you can add on a Reverse T3 and Vitamin D test.
In the meantime, if you have any form of Hashimoto’s thyroiditis, your thyroid health is best managed by an integrated team of healthcare professionals. Our healthcare providers at Paloma Health are dedicated to diagnosing and managing your thyroid condition for optimal health and wellness!