Discussion Many reports indicate the usefulness of Tg concentration measurement before radioiodine treatment to early detection of DTC relapse or metastasis [4-8, 20, 21]. Three factors determine Tg concentration in most clinical situations: thyroid cell mass, thyroid cell damage and activation of TSH receptors [22]. When Tg is measured before radioiodine ablation the effects of surgical damage are generally vanished and endogenous TSH levels are increased in all patients: consequently the thyroid remnant mass is the major determinant of the serum Tg concentrations [23]. However, the rhTSH-stimulated Tg cannot be used instead of preablative offT4-Tg when rhTSH is employed to prepare radioiodine ablation [12]. Therefore we evaluated the role of post-surgery onT4-Tg as surrogate prognostic marker. We choiced to suppress TSH levels in order to normalize the effect of TSH stimulation on thyroid remnants. Clearly, the TSH suppression reduced Tg levels: however, the high-sensitive Tg assays provide a good distinction between the lower limit of the euthyroid reference range and the functional sensitivity limit detecting small amounts of thyroid tissue even in the TSH-suppressed state (22, 23). We showed a significant positive relationship between post-surgery Tg measured during T4 treatment and after TSH stimulation. No relationship was found between RAIU (i.e. expression of remnant mass) and onT4-Tg, probably due the clustering of all Tg levels lower than 0.2 ng/mL (see statistics paragraph). However, undetectable onT4-Tg after surgery identifies patients free of metastasis at PT-WBS and without late recurreces during early 12-months follow-up. None of these patients showed a significant increase in both rhTSH and offT4-stimulated Tg before radioiodine ablation. This seems to indicate that the relationship between Tg expression and thyroid tissue mass is manteined even in TSH suppression state: Therefore undetectable serum onT4-Tg really identifies patients without significant thyroid tissue amount as well as stimulated Tg. All patients performed extracapsular total thyroidectomy in a dedicated thyroid surgery unit and the thyroid remnant, expressed as RAIU, was lower in our series than in others. This means that our data cannot directly translated to patients treated by more limited surgery. Globally, basing on our data we conclude that preablative onT4-Tg may be of value as prognostic marker when rhTSH-aided radioiodine ablation is done. Additionally, the role of preblative onT4-Tg measurement as a yard-stick for radioiodine ablation should be further evaluate.