De Quervain thyroiditis is diagnosed through clinical and test results, with laboratory features including elevated C-reactive protein and erythrocyte sedimentation rate.

Tissue diagnosis is rare, but fine needle aspiration may be helpful in questionable cases to differentiate unilateral involvement from bleeding into a cyst or tumor.

[2] The neck pain is usually unilateral at first, then spreads to the other side and can radiate to the ipsilateral jaw, ear, occiput, or chest.

[2] Approximately half of affected adolescents and two-thirds of adults have widespread thyroid gland involvement.

[2] During the early stages of the condition, almost half of patients experience thyrotoxic symptoms such as anxiety, tachycardia, palpitation, and weight loss.

[7] De Quervain's thyroiditis has been linked to a variety of viral illnesses, including mumps,[9] adenovirus,[10] Epstein-Barr virus,[11] cytomegalovirus,[12] coxsackievirus,[13] influenza,[10] echovirus,[14] and enterovirus.

[2] About two thirds of patients with de Quervain thyroiditis were found to have positive histocompatibility antigen (HLA) B35 results.

[16][17] Furthermore, it was found that identical twins who were heterozygous for the HLA B35 haplotype also developed de Quervain thyroiditis at the same time.

[23] Thyroid scans using technetium (99mTc) pertechnetate or RAIU usually show minimal uptake during the acute phase.

[2] Severely sick individuals may benefit from glucocorticoid medication, which often produces a substantial response in 24 to 48 hours.

[3][5] Regardless of the severity of the disease or the type of treatment used, 20–56% of adult patients experienced transient hypothyroidism a few weeks after the hyperthyroid period.

[6][28] Recurrent de Quervain thyroiditis is rare, occurring in about 2% of cases, and usually manifests again within a year after diagnosis.