Lessons of the month 1: A diagnostic dilemma: thyroid abscess or subacute thyroiditis?

Discussion

Subacute thyroiditis is a thyrotoxic condition characterised by neck pain, fever and a tender diffuse goitre, commonly seen in young and middle-aged adults with higher incidence in women.² It was first defined by Swiss surgeon de Quervain in 1902 as a self-limiting inflammatory disorder of the thyroid gland.³ A post-viral inflammatory response in subacute thyroiditis leads to giant cell infiltration into the thyroid follicles. This results in follicular swelling and stretching of the capsule causing pain and tenderness, and follicle disruption with release of stored thyroid hormone into the circulation leading to hyperthyroidism.⁴ Although subacute thyroiditis often presents with tender diffuse goitre, there are exceptions, such as in our patient, as well as the case of a patient with a painful thyroid nodule reported by Li et al.⁵

Thyroid abscess is most often seen in children with local anatomic defects, such as a pyriform sinus fistula or third and fourth bronchial arch anomalies.^6,7 In adults, it is seen in patients with Hashimoto's disease, large goitre or thyroid cancer, and people who are immunocompromised, older or debilitated. It could also arise via haematogenous or lymphatic spread or from iatrogenic infections after FNA.^6,7 The clinical manifestations in both subacute thyroiditis and thyroid abscess can be similar, although 83.1% of the patients with bacterial suppurative thyroiditis were euthyroid.⁸ Both diseases show markedly elevated erythrocyte sedimentation rate (ESR) and CRP. The WBC is normal or slightly elevated in subacute thyroiditis but is significantly elevated in a thyroid abscess.⁶ Due to the overlapping clinical features, it was difficult to make the initial diagnosis in this patient. She had high-grade fever and a tender thyroid mass, with an elevated WBC and CRP. Antibiotic therapy was therefore started on the presumptive diagnosis of thyroid abscess and NSAIDs were given for pain relief. The most common cause of thyroid abscess is acute bacterial infection with Staphylococcus and Streptococcus species being the major pathogens, while mycobacterial and fungal infections tend to be more chronic and common in immunocompromised patients.^1,8,9

The mainstay of treatment in subacute thyroiditis is to suppress the thyroid inflammation with salicylates and other NSAIDs. In severe cases, steroids are the most effective therapy.⁴ In thyroid abscess, the key is to treat the infection with antibiotics. If antibiotic treatment fails, surgical drainage and/or thyroid surgery is necessary. In this case, NSAIDs were started early to alleviate her symptoms and thus benefited her when the diagnosis was revised to subacute thyroiditis. As a thyroid abscess has the potential to progress very rapidly, inadequate treatment can be fatal, resulting in mortality of over 12%.^7,8

Ultrasound is usually sufficient to identify a thyroid abscess as it is easily accessible, but in certain cases it may not be obvious.⁹ Heterogeneous and hypoechoic features, as described in our case, are rather non-specific.¹⁰ It is also difficult to differentiate subacute thyroiditis from a thyroid abscess using a thyroid uptake scan as both may show low uptake.⁹ Computed tomography is rarely needed unless ultrasound is inconclusive or local anatomic defects are suspected. Hence, clinical judgement, cytopathologic findings from FNA and cultures may be helpful in differentiating both. Presence of giant cells, granulomas, inflammatory cells (lymphocytes, macrophages and neutrophils), degenerated follicular epithelial cells, with a background of thick colloid with cellular debris are the key cytological characteristics of subacute thyroiditis, while there will be aspiration of pus in thyroid abscess.^6,10 Imaging studies on our patient revealed the possibility of either one of the two diseases. By doing FNA, we were able to conclude the diagnosis of subacute thyroiditis. She eventually recovered with NSAIDs and beta blockers.