The normal TSH, or thyroid-stimulating hormone, level in the blood is 0.4 to 4.2 microunits per milliliter, according to WebMD. In children, the normal TSH level is 0.7 to 6.4 microunits per milliliter. For newborns, a normal level of TSH is 1 to 39 microunits per milliliter.
High TSH levels often result from hypothyroidism, which is an underactive thyroid, notes WebMD. Hashimoto’s thyroiditis and pituitary gland tumors also cause elevated TSH levels. Low TSH levels often result from hyperthyroidism, or an overactive thyroid. Other causes include Graves’ disease, noncancerous tumors and pituitary gland damage. Women often experience low TSH levels during the first trimesters of their pregnancies.