Thyroid stimulating hormone (TSH) is the medical gold standard for testing thyroid function. TSH is released by the pituitary gland in the brain and its role is to tell the thyroid to produce hormones and release them into the bloodstream. It is not measuring thyroid hormone itself. You are typically considered hypothyroid only when your TSH is high. However, there are many other situations resulting in low thyroid function with a low or normal TSH. I believe testing TSH alone to assess the thyroid is a huge medical problem causing unnecessary suffering to many.
If your TSH is normal it doesn't necessarily mean you have a normal thyroid!