Thyroid Stimulating Hormone (TSH), is a pituitary hormone that stimulates the thyroid gland to produce T3 (triiodothyronine) and T4 (thyroxine). TSH consists of an alpha and beta subunit; the alpha subunit is highly similar to the alpha subunit of HCG, LH and FSH, while the beta subunit is unique to TSH and determines specificity for the TSH Receptor. TSH concentrations are measured as part of thyroid function tests and may be monitored on an ongoing basis.