In cases where the infection is thought to be associated with a sinus fistula it is often necessary to confirm the presence of the fistula through surgery or laryngoscopic examination. While invasive procedures can often tell definitively whether or not a fistula is present, new studies are working on the use of computed tomography as a useful method to visualize and detect the presence of a sinus fistula. Diagnostic tests
Fever, redness, swelling
Pain
Blood tests of thyroid functions including TSH, T4 and T3 are usually normal 
Ultrasonographic examination often shows the abscess or swelling in thyroid
Gallium scan will be positive
Barium swallow will show fistula connection to the piriform sinus and left lobe
Elevated white blood cell count
Elevated erythrocyte sedimentation rate
Fine-needle aspiration

Subtypes of thyroiditis
Treatment
Treatment of AIT involves antibiotic treatment. Based on the offending organism found on microscopic examination of the stained fine needle aspirate, the appropriate antibiotic treatment is determined. In the case of a severe infection, systemic antibiotics are necessary. Empirical broad spectrum antimicrobial treatment provides preliminary coverage for a variety of bacteria, including S. aureus and S. pyogenes. Antimicrobial options include penicillinase-resistant penicillins (ex: cloxacillin, dicloxacillin) or a combination of a penicillin and a beta-lactamase inhibitor. However, in patients with a penicillin allergy, clindamycin or a macrolide can be prescribed. The majority of anaerobic organisms involved with AIT are susceptible to penicillin. Certain Gram-negative bacilli (ex: Prevotella, Fusobacteriota, and Porphyromonas) are exhibiting an increased resistance based on the production of beta-lactamase. Patients who have undergone recent penicillin therapy have demonstrated an increase in beta-lactamase-producing (anaerobic and aerobic) bacteria.
Acute infectious thyroiditis (AIT) also known as suppurative thyroiditis, microbial inflammatory thyroiditis, pyrogenic thyroiditis and bacterial thyroiditis.The thyroid is normally very resistant to infection. Due to a relatively high amount of iodine in the tissue, as well as high vascularity and lymphatic drainage to the region, it is difficult for pathogens to infect the thyroid tissue. Despite all this, a persistent fistula from the piriform sinus may make the left lobe of the thyroid susceptible to infection and abscess formation. AIT is most often caused by a bacterial infection but can also be caused by a fungal or parasitic infection, most commonly in an immunocompromised host. Signs and symptoms
In most cases AIT is characterized by onset of pain, firmness, tenderness, redness or swelling in the anterior aspect of the neck. Patients will also present with a sudden fever, difficulty swallowing and difficulty controlling the voice. Symptoms may be present from 1 to 180 days, with most symptoms lasting an average of about 18 days. The main issue associated with the diagnosis of AIT is differentiating it from other more commonly seen forms of thyroid conditions. Pain, fever and swelling are often much more severe and continue to get worse in people who have AIT compared to those with other thyroid conditions. Causes
Despite the thyroid gland being extremely resistant to infection, it is still susceptible to infection by various bacteria. The cause can be almost any bacterium.