Thyroid hormone is generally accepted in the treatment of patients with well-differentiated thyroid cancer, a tumor that is known to be capable of growth in the presence of high thyroid-stimulating hormone (TSH) levels. Using 'TSH suppression' as the endpoint in treatment, the dosage of levothyroxine (L-T4) is often adjusted to achieve subnormal or undetectable TSH levels. Still, the effect of this therapy on the cancer and the health maintenance of the individual patient remains inadequately defined. This is confounded by the fact that advances in TSH measurement technology have made widely available commercial assays that can detect TSH from 1-3 orders of magnitude below normal euthyroid levels. As such, an 'undetectable' TSH from one laboratory or study may not be the same as an 'undetectable' TSH from another laboratory. It therefore behooves the clinician to understand the risks in the individual patient for progression or death from the cancer, the expected cancer response to thyroid hormone therapy, and the potential complications resulting from different levels of thyroid hormone treatment. Further study is needed to precisely establish the effects of TSH 'suppression' on different parameters of thyroid cancer response.