In hyperthyroidism, if part of the thyroid is removed, does this decrease the thyroid hormone in the body?

Yes, but...: If a person with an overactive thyroid producing too much hormone, then removal of half of it will leave behind a smaller hormone-producing gland. Hormone levels might still be abnormal. The remaining gland could make too much or too little hormone, depending on how much total activity remains.

Depends on the cause: Depends on cause. Surgery for hyperthyroidism is either graves or hot nodule. Removing hot nodule will normalize thyroid levels. Surgery for graves is a total thyroidectomy and will require thyroid hormone replacement.