Background: On April 19, 2022, the Thailand Department of Disease Control was notified of a cluster of thyrotoxicosis among inmates in a Northeastern provincial prison. We investigated to confirm the diagnosis and the outbreak, describe epidemiological characteristics, identify the possible source, and provide recommendations.

Methods: In a descriptive study, all inmates and prison staff were screened by history taking and physical examination. Suspected cases were inmates and staff who had at least one of the following symptoms or signs: palpitation, shaking hand, agitation, excessive sweating, tachycardia, hand tremor, and limb weakness during April 1-22. Confirmed cases were those whose thyroid function test results showed low serum thyrotrophin (TSH) with normal or elevated serum levels of triiodothyronine (T3) and free thyroxine (T4). The cooking process and food preparation were assessed. The cases’ blood samples, leftover meat in the prison, and outside meat samples were tested for thyroid hormone levels.

Results: Of 3,052 inmates, 214 (7.0%) thyrotoxicosis cases were identified. No staff reported illness. Seven inmates were hospitalized. Male to female ratio was 1.7:1. Wing 8 had the highest attack rate (17%), followed by Wing 5 (7.5%) and Wing 7 (7.4%). The major symptoms/signs were headache (71%), myalgia (63%), and fatigue (63%). Among the confirmed cases, thyroglobulin level was low to normal. Cooked chicken and pork were the most predominant meal and were served almost every day in April. During the outbreak, some chicken packages were mixed with unidentified parts including internal organs. Cooked chicken and chicken balls showed elevated thyroid hormone levels, compared to cooked chicken from an outside local market.

Conclusions: The thyrotoxicosis outbreak among inmates was confirmed. Chicken meat and meatballs were the potential sources. The prison strengthened food safety inspection before cooking and engaged with health volunteers for illness surveillance and early detection, and the outbreak subsided after that.