In simple words: Tuberculosis (TB) can sometimes affect the thyroid gland, although it's rare. This happens when TB bacteria spread from the original infection site, often in the lungs, to the thyroid through the bloodstream. It can cause symptoms like hoarseness, difficulty swallowing or breathing, fever, weight loss, and neck swelling.

Tuberculosis of the thyroid gland is a rare manifestation of extrapulmonary tuberculosis. It results from reactivation of Mycobacterium tuberculosis or, rarely, Mycobacterium bovis bacilli lodged in the thyroid gland, typically having migrated there through the bloodstream from an original infection site. Patients may experience symptoms such as a hoarse voice, difficulty swallowing (dysphagia), difficulty breathing (dyspnea), and rarely, recurrent laryngeal nerve paralysis.Typical TB symptoms like night sweats, fever, weight loss, and localized pain and inflammation are also possible. Diagnosis involves blood tests, tuberculin skin test, imaging (ultrasound, CT, or MRI), and fine needle aspiration cytology (FNAC) to identify acid-fast bacilli.

Example 1: A 55-year-old male presents with an 8-week history of midline neck swelling and 2 weeks of pain in the swollen area. FNAC reveals granulomas with epithelial and giant cells against a caseous background, consistent with tuberculosis of the thyroid gland., A patient with a history of pulmonary tuberculosis develops a hoarse voice, difficulty swallowing, and a palpable thyroid nodule. Imaging and FNAC confirm the presence of Mycobacterium tuberculosis in the thyroid gland., A patient presents with symptoms mimicking acute bacterial thyroiditis, including neck pain, fever, and tenderness over the thyroid. Further investigation reveals subclinical thyrotoxicosis and the presence of tuberculous granulomas in the thyroid gland.

Documentation for A18.81 should include evidence of active tuberculosis infection, such as positive AFB smear or culture, histological findings of caseating granulomas, and imaging findings consistent with thyroid involvement. Patient's history, presenting symptoms, and results of other relevant diagnostic tests, such as blood tests and tuberculin skin test should also be documented.