Discussion

This case is a unique presentation of coexisting thyroiditis and giant cell arteritis, a combination that has been described only 6 times previously in the literature ( Table 3 ).

This unusual condition may be explained in several ways.

First, this case may represent the co-occurrence of 2 rare diseases in the same patient.

Second, it is possible that the same pathophysiology underlies both diseases, possibly triggered by a prior viral infection or other precipitating factor. It should be noted that both disorders share similar histological and clinical patterns. The elevated T4, suppressed TSH, tender thyroid gland, abnormal hypoechogenicity of the thyroid on ultrasound examination, and low RAIU support the diagnosis of subacute thyroiditis. The clinical presentation of subsequent headache, transient monocular blindness, and diplopia, together with the abnormal temporal artery biopsy results, support the diagnosis of temporal arteritis.

A third explanation is that vasculitis may involve the arteries supplying both the scalp (as in the classic forms of giant cell arteritis) and the thyroid. The common carotid artery is the source of arterial branches supplying blood to both tissues. Giant cell arteritis may involve any large or medium-sized artery in the body, including the thyroid arteries. If this pathophysiologic mechanism can be confirmed by a combination of clinical features and histology, it may be useful to add to the classification of thyroiditis a new category: "thyroiditis associated with giant cell arteritis." Glucocorticoids would be the treatment of choice in this instance. Although a surgical biopsy of the thyroid was not performed in our patient, the working hypothesis of both the rheumatology and endocrinology consultants is that she represents a case of vasculitis of the thyroid arteries, which produced inflammation, localized ischemic necrosis, and pain in the thyroid gland.

Some evidence indicates that there is an increased incidence of other autoimmune disorders in patients with hypothyroidism, particularly that due to Hashimoto's thyroiditis. One study showed that 13.7% of in-patients followed for disorders such as lupus and Sjögren's syndrome developed hypothyroidism with positive thyroid antibodies, consistent with Hashimoto's thyroiditis. There is also a putative association between hypothyroidism with positive thyroid antibodies and polymyalgia rheumatica (PMR), but recent studies have shown no association between these 2 conditions, despite the fact that they manifest common symptoms and patients with PMR also have a high prevalence of circulating thyroid antibodies.

Both thyroiditis and giant cell arteritis should be included in the differential diagnosis of fever of unknown origin. Our experience suggests that if one disorder is diagnosed, the other should be considered as a potential coexisting disease. Further studies are needed to investigate whether vasculitis of the thyroid artery represents a distinct category in the classification of thyroiditis.

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