TSH (Thyroid Stimulating Hormone) is made in the pituitary gland (brain appendage) to stimulate the thyroid gland to produce thyroid hormone. An overly high TSH fits a poorly functioning thyroid, a low TSH fits an overly functioning thyroid.
Note during pregnancy different reference values apply:
1st Trimester 0.33 - 4.59 mU/L
2nd Trimester 0.35 - 4.10 mU/L
3rd Trimester 0.21 - 3.15 mU/L
Normally the values of someone over 20 are between 0.27 and 4.20 mU/L