J. Endocrinol. Invest. 18.'165-166,1995 Post-surgical follow-up of differentiated thyroid cancer F. Pacini, R. Elisei, L. Fugazzola, F. Cetani, C. Romei, F. Mancusi, and A. Pinchera Istituto di Endocrinologia, University of Pisa, Italy INTRODUCTION Metastases of well differentiated thyroid cancer (DTC) retain some differentiation function of the nor- mal follicular cells, mainly iodine uptake and thy- roglobulin (Tg) synthesis and secretion. These properties have been used to develop diagnostic tools - whole body scan (WBS) with 131 1 and serum Tg measurement for monitoring patients with DTC after thyroidectomy. This article will review the methodological and clinical problems associated with the follow-up of these patients. WHOLE BODY SCAN Iodine uptake in metastatic thyroid cancer requires high stimulation of the cells by endogenous TSH to be visualized by scanning devices. Thus, patients must be rendered hypothyroid before performing WBS. The usual time required to induce sufficiently high levels of TSH is 45 days withdrawal for L-thy- roxine (L -T 4) and 15 days for L-triiodothyronine. Some authors have used administration of exogenous bovine TSH to promote 131 1 uptake from metastatic tissue, but this procedure has been abandoned be- cause it may be associated with allergic reaction, neutralizing anti-TSH antibodies and because the ef- ficiency with respect to endogenous TSH is much lower. Initial trials using recombinant TSH seem promising in inducing efficient levels of uptake in the tumor avoiding the unpleasant effect of hypothy- roidism to the patient. The choice of the amount of 131 1 to be given as trac- er dose for WBS has been a matter of controversy. Tracer doses as low as 0.2-0.5 mCi have been as- sociated with too much falsely negative WBS. On the other hand, larger diagnostic doses of 131 1 ac- tivity may produce a sublethal radiation effect on the metastases which is enough to prevent subse- quent uptake of the therapeutic 131 1 administration, but not enough to kill the cell. Based on these con- siderations, the optimal scanning dose of 131 1 is comprised between 2 and 5 mCi. Key-words.' Thyroid cancer, follow-up, thyroglohulin, whole body scan. Correspondence: Dr. F. Pacini, Istituto di Endocrinologia, Viale del Tirreno 64, 56018 Tirrenia, Pisa. Italy 165 Another methodological problem to consider, is the effect of iodine contamination as source of falsely negative WBS. Many iodine containing foods and drugs may be inadvertently assumed by the patient before performing WBS. Since in our experience such possibility is not rare, it is important to inform the patient and the family doctor to avoid iodine containing food or medications before WBS. If pos- sible, we also recommend routine measurements of urinary iodine excretion to rule out iodine con- tamination. After surgery, the presence of 131 1 up- take in the thyroid bed when performing the first WBS, is the rule. Depending on its extent, it may prevent 131 1 uptake of metastatic tissue elsewhere in the body. It is our policy, and of many other au- thors, to ablate any thyroid residue with therapeu- tic doses of 131 1 (30-50 mCi). SERUM TG MEASUREMENT The introduction of serum Tg measurement. some 20 years ago (1), has greatly enhanced the man- agement of patients with DTC. After thyroidectomy there is a very good correla- tion between serum Tg concentrations and pres- ence of persistent or recurrent disease. Residual thyroid tissue is associated with low-moderate lev- els of circulating Tg, while high Tg levels are almost invariably present in patients with well differentiat- ed metastases from DTC. The higher Tg elevations are found in patients with lung or bone metastases, but also lymph node metas- tases express significant amount of serum Tg (2, 3). Tg secretion is under TSH control. SuppreSSion of endogenous TSH by L-T4 therapy decreases the levels of serum Tg with respect to off L-T4 values. However, even on L-T4, serum Tg levels remain de- tectable in the case of distant metastases, while may become undetectable in the case of node metastases. Thus, serum Tg results must be con- sidered with caution in patients studied on L-T4 (4). CORRELATION BETWEEN SERUM TG AND WBS Usually, a good correlation is found between the re- sults of serum Tg and those of WBS (5). Patients with positive WBS have almost invariably detectable