When thyroid glands are completely removed, permanent hypothyroidism develops in the patient.  Synthetic thyroid hormone is replaced from outside of the body to prevent this condition. Until thyroid hormone levels are balanced with synthetic hormones and reach normal levels, the patient may gain weight.

Patients may gain more weight because of reducing their movement and not taking care of their diet in the postoperative recovery period.  However, if the weight gain is more than 5 kg, then the cause is not hypothyroidism.

Radioactive iodine treatment is administered to some patients with thyroid cancer. The level of TSH should be above 30 for an effective radioactive iodine treatment. T his is a serious hypothyroidism condition.  If the patient does not take care about his diet during this period, weight gain is inevitable.  Thyroid hormone replacement is started after radioactive iodine treatment and within 1-2 months the patient's weight returns to normal.

Usually very severe hypothyroidism is not seen in Hashimoto's disease. The weight gain complaints of patients with TSH levels of 5-10 mIU/L cannot be explained with Hashimoto's disease.