I don't understand your question. TSI and T4 are 2 different tests; TSI is a test to determine if one has Graves Disease, while T4 is a test to determine thyroid hormone level. Can you please verify which test you're asking about?
If you have the actual results of your test, please post them and be sure to include reference ranges, which vary lab to lab and have to come from your own report.
If it's TSI, Thyroid stimulating immunoglobulins, these are antibodies that can cause hyperthyroidism (graves disease). Low is good.
T4 is one of the thyroid hormones. The other is T3. Low T4 points to hypothyroidism. If you post your lab results and ranges, someone will comment on them.