TSH (aka Thyroid-Stimulating Hormone, Thyrotropin) is the main regulator of thyroid hormone biosynthesis and secretion. TSH reference limits increase with age and vary somewhat between racial groups. Although daily levels can fluctuate a little (due to sleep deficiency or severe illness for example), they are generally constant and therefore are critical determinants of thyroid function and thyroid health.
The pituitary gland at the base of the brain produces thyroid-stimulating hormone, which tells the thyroid gland how much T3 and T4 to produce. The TSH level in your blood reveals how much T3/T4 your pituitary gland is asking your thyroid gland to make. If your TSH levels are abnormally high, it could mean you have an under-active thyroid, or hypothyroidism.