RESUMEN
De-differentiated thyroid carcinoma is characterized by loss of thyroid-specific functions and properties. The therapeutic options for this type of thyroid cancer are limited and generally not efficient. Recent studies with retinoic acid (RA) have shown that this drug can induce re-differentiation of the thyrocyte and tumor regression after 131I therapy. The aim of the present study was to assess the effects of RA therapy in patients with extensive thyroid tumor involvement, which lost radioiodine uptake ability. A total of 5 patients (1 follicular carcinoma, 3 papillary carcinomas and 1 poorly differentiated carcinoma) were treated with isotretinoin (1.0 to 1.5 mg/kg/day) for 5 weeks and then submitted to radioiodine therapy. Three parameters for assessment of RA effects were established: a) reduction of serum thyroglobulin levels; b) increment of the post-therapeutic dose radioiodine uptake; c) tumor size regression after therapy. All patients completed the treatment and the most frequent side effects were dry skin and lips and hypertriglyceridemia. One patient showed satisfactory response (2 or more of the 3 criteria were reached) and a new cycle of RA was given. In two, just a partial response (1 criterion) was seen and the other patients did not respond. Based on these results, isotretinoin might be an option for de-differentiated thyroid cancer, with low rate of severe side effects, especially when compared with cytotoxic drugs. Aggressive thyroid cancer frequently needs multimodal adjuvant therapy.