THYROID STIMULATING HORMONE (TSH), SERUM The TSH Serum Test, or Thyroid-Stimulating Hormone Serum Test, is a blood test that measures the concentration of TSH in the bloodstream. TSH is produced by the pituitary gland and regulates the thyroid gland's production of thyroid hormones (T3 and T4). This test is a key indicator of thyroid function and helps assess whether the thyroid is working properly.