A TSH (thyroid-stimulating hormone) test is a simple blood test that measures how well your thyroid gland is working.

Your thyroid is controlled by a gland in your brain called the pituitary gland. The pituitary gland makes thyroid-stimulating hormone (TSH). TSH tells your thyroid how much thyroid hormone it needs to make. 

If the thyroid hormone level in your blood is too low, Your thyroid is underactive, as in hypothyroidism, your pituitary gland makes larger amounts of TSH to tell your thyroid to work harder.  Not having enough thyroid hormone slows down your body functions and causes symptoms that can vary from person to person.

 If your thyroid hormone level is too high, Your thyroid is overactive, as in hyperthyroidism,  the pituitary gland makes little or no TSH. Having more thyroid hormones than you need speeds up your body functions and causes symptoms that can vary from person to person.

 By measuring the TSH level in your blood, you can find out if your thyroid is making the right level of hormones.

So a TSH Test is needed, A TSH test is used to find out how well your thyroid is working. It can tell if you have hyperthyroidism (too much thyroid hormone) or hypothyroidism (too little thyroid hormone) in your blood.