Sjögren syndrome and autoimmune thyroiditis
Although autoimmune diseases are labeled as separate diagnostic entities, it's important to bear in mind that this nomenclature masks the fact that they are like branches growing from the same trunk. While the tissue targets of autoimmune inflammatory attack differ according to the individual, the underlying causal factors are similar. Moreover, if one tissue is under autoimmune attack, it is almost always that others are too. A report published in the Journal of Clinical Rheumatology draws attention to the association of autoimmune thyroid disease and Sjögren syndrome. The authors state:
"The thyroid, salivary, and lacrimal glands are susceptible to immunologic damage, which can be expressed as an organ-specific autoimmune disease such as thyroiditis, or a systemic autoimmune disease such as primary Sjögren syndrome (pSS). Sjögren syndrome is characterized by the progressive destruction of the exocrine parotid and lacrimal glands, causing mucosal and conjunctival dryness (sicca syndrome). The serology of patients with pSS often shows elevated levels of antibodies including antinuclear antibody, rheumatoid factor, Ro (SS-A), and La (SS-B). There is a growing body of literature suggesting an increased risk of autoimmune thyroid disease (AITD) in individuals with pSS and an increased risk of pSS in individuals with AITD."
They continue to describe the case of a 41-year-old man suffering from autoimmune hypothyroid disease who was also found to have pSS after thyroid hormone replacement therapy was initiated. They elaborate the salient implications:
"Patients with one autoimmune disease are often at increased risk for developing another autoimmune disease. In the case of 170 Hungarian subjects with Hashimoto thyroiditis (HT), 17% had Sjögren syndrome. In a group of 176 Spanish patients with AITD...the prevalence of keratoconjunctivitis was 23% and that of xerostomia was 37%. In 2 smaller studies of patients with autoimmune thyroiditis and hypothyroidism, 22% to 58% had salivary gland abnormalities as demonstrated by parotid scintigraphy, sialometry, and/or salivary gland biopsies showing lymphocytic involvement."
Furthermore...
"...patients with pSS may show higher rates of AITD. The largest retrospective study to date included 479 patients from Hungary with pSS, who had thyroid function testing every 3 to 6 months. The frequency of HT was 6%, which was greater than the 1% to 2% frequency in the general population...In other smaller studies, the prevalence of HT in patients with pSS ranged from 11% to 50%."
The authors note a study in which the sera from 26 patients with pSS and 7 patients with hypothyroid were tested for antihuman thyroglobulin (antihTg) antibody activity. Interestingly, all of the pSS sera contained IgG and IgM antihTg autoantibodies, the antihTg autoantibodies in patients with pSS and HT overlapped in their reaction with a certain region on the thyroglobulin molecule. They conclude:
"As demonstrated in this case, primary Sjögren syndrome and AITD are often associated. Although not all studies consistently demonstrated an increased association, the number of positive studies and the larger studies suggest a true association...Rheumatologists and endocrinologists may miss the presence and association of these 2 autoimmune conditions, given several nonspecific and overlapping symptoms such as fatigue, weight gain, and diffuse myalgias. We therefore recommend that rheumatologists consider assessing patients with Sjögren syndrome with periodic thyroid function testing."
Moreover, clinicians managing any autoimmune disease should be alert to the presence of autoimmune activity involving additional target sites. And of course we must bear in mind the necessity of going beyond symptom management to investigate and treat the underlying causal factors. Those with a particular interest may wish to read the earlier post on Sjögren syndrome.