Brief Answer:
Thyroid

Detailed Answer:
I do follow your question.

So even if half your thyroid is removed by surgery, there is a good chance you will not ned to take thyroid medication such as levo thyroxine for the rest of your life, provided the remainder of your thyroid is functioning normally.

Some people have a condition called Hashimoto's thyroiditis in which the thyroid does not work well and consequently does not make enough quantities of thyroid hormone. These individuals develop 'hypothyroidism' ie an under-active thyroid. They typically need thyroxine for the rest of their lives because this condition tends to be irreversible and permanent.

So if your TSH before surgery was normal and not abnormally high, it is unlikely you have this problem.
There are other ways to determine too whether or not you have Hashimoto's thyroiditis. One is physical examination and the other is blood testing for thyroid auto antibodies :Anti Thyroid Peroxidase (TPO) antibodies and Anti Thyroglobulin antibodies.

But if the TSH is normal before surgery, it is not standard practice to test for these.

So in summary, even much less than half the thyroid gland is sufficient to produce required amounts of thyroid hormone, provided it is functioning well and not affected by any disease process.

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