Patient Li Moumou, male, 43 years old. He had a history of hyperthyroidism and had been treated with regular anti-thyroid medication for 1.5 years and had stopped taking it for 5 years. He denied the history of hepatitis and was found to be negative for hepatitis virus antibodies. Physical examination: clear consciousness, no proptosis, thyroid gland II enlarged, soft, no pressure pain, no mass, swallowing activity, heart rate 90 beats/min, rhythmical, hands positive for fine tremor. The local hospital recommended consultation for 131I therapy. Clinical analysis: In this case, the patient was referred for consultation of 131I therapy due to recurrence of hyperthyroidism combined with abnormal liver function, and there were indications for 131I therapy. The dose of 131I treatment was 9 mCi, and the liver function was basically normal on recheck 2 weeks after treatment. The thyroid function was normal at TT3, TT4, FT3, FT4, and TSH 13.318lmIU/L, suggesting subclinical hypothyroidism, and he was given 12.5 mcg/day of oral Eugenol tablets. The thyroid function was normal in TT3, TT4, FT3, FT4 and TSH 0.08lmIU/L. Therefore, the patient stopped taking Eugenol tablets and was advised to review the thyroid function after 2 months. One year after treatment, the patient’s thyroid function, blood count and liver function were normal. It is recommended to continue regular review (once a year). Discussion: Fatigue, drowsiness, increased body mass, chills, convulsions, increased menstrual flow and constipation in young and middle-aged women after 131I treatment for hyperthyroidism suggest the possibility of early onset hypothyroidism and should be reviewed promptly. Early onset hypothyroidism should be considered when thyroid hormone levels are below the normal range, even if TSH is still suppressed. Replacement therapy with levothyroxine sodium tablets or thyroid tablets is available after the onset of hypothyroidism. The timing of replacement therapy depends on thyroid function tests, clinical symptoms and physical examination findings. The dosage should be adjusted under the guidance of a physician to maintain normal thyroid hormone levels, and regular follow-up reviews should be insisted upon during replacement therapy.