Hashimoto's thyroiditis is commonly associated with goiter in approximately 69.44% of cases. Most patients present as euthyroid or hypothyroid, while just a small percentage of patients present in the hashitoxicosis phase. The clinical presentation may become more complex when additional comorbidities are present. Chronic diseases such as tuberculosis can alter the clinical manifestations of Hashimoto's thyroiditis, thereby presenting diagnostic challenges for clinicians.

A 6-year-old male patient presented with goiter, weight loss, malnutrition, and hyperhidrosis. The patient had a history of tuberculosis and was in the first month of active phase therapy. Anthropometric assessment revealed a weight of 14 kg, height of 109 cm, and a BMI for age score of –3.45 SD, indicating malnutrition. Vital signs included a pulse of 90 beats per minute, afebrile status, blood pressure of 110/60 mmHg, and a normal respiratory rate. Additional clinical findings included a fine tremor without exophthalmos. Hashitoxicosis was initially suspected and laboratory evaluation was performed. Results indicated subclinical hypothyroidism with fT4: 0.9 ng/dL (normal: 0.7–1.8), TSH: 22.4 μIU/mL (normal: 0.3–5.0), and anti-thyroid peroxidase (anti-TPO) antibodies >1000 U/m (normal: <80). Additionally, the thyroid uptake test showed normal uptake. The diagnosis was changed to Hashimoto's thyroiditis with subclinical hypothyroidism, and levothyroxine substitution dose was initiated in conjunction with anti-tuberculosis therapy.

The diagnosis of Hashimoto's thyroiditis was confirmed by elevated anti-TPO. Although patients may present with symptoms resembling thyrotoxicosis, a pulse rate of 90 beats per minute, and laboratory findings indicate subclinical hypothyroidism. In this case, the final diagnosis was Hashimoto's thyroiditis with subclinical hypothyroidism. The diagnostic complexity of this case comes from discrepancies among clinical symptoms, physical examination, and laboratory data. Thyroid hormone levels are not sufficiently reduced to cause hypothyroid symptoms. The presence of active tuberculosis, which produces systemic symptoms, can mask subclinical hypothyroid manifestations and mimic those of Hashitoxicosis, consequently increasing the risk of misdiagnosis.

Active tuberculosis can mask the clinical manifestations of Hashimoto's thyroiditis, complicating the identification of thyroid disease. In conditions like this, laboratory examinations play a critical role in guiding the treatment of Hashimoto’s thyroiditis.