​Thyroid Stimulating Immunoglobulin (TSI)
Produced by: Lymphocytes (white blood cells)
Definition / Purpose: TSI is a form of immunoglobulin G (IgG) that can bind to TSH receptors on the thyroid gland. TSI mimics the action of TSH, causing excess secretion of T4 and T3. TSI is not regulated by the normal feedback loop associated with TSH that is produced in the pituitary gland.
Lab Reference Range: Normal levels are < or =1.3 TSI index or < or =130%
When to order:

• With patients that are hyperthyroid or have symptoms of hyperthyroidism and are not on thyroid hormone replacement therapy.

Pearls:
Half-life: Approximately 5 days.
TSI is an autoantibody.
TSI is present in Graves’ Disease and Thyrotoxicosis.
Unlike TSH, there is no feedback mechanism to slow down the production of TSI.