RESUMEN
Serum thyroglobulin and cervical ultrasonography are the milestones of the follow up of patients with differentiated thyroid carcinoma. When levels of thyroglobulin, stimulated either by discontinuing thyroid hormone supplementation or by using human recombinant TSH are undetectable and cervical ultrasonography is negative for relapse, there is a 99 percent probability that the patient is free of disease. Twenty percent of patients with undetectable thyroglobulin levels under thyroid hormone supplementation, will have levels above 2 ng/ml when treatment is discontinued and in one third of them, a relapse will be detected. Pre ablative thyroglobulin levels below 27.5 ng/mg have a positive predictive value of 98 percent for a disease free survival in low risk patients. Anti thyroglobulin antibodies must be measured along with thyroglobulin value, rendering false negative results. Thyroglobolin determination in the needle washout is useful, when a suspicious cervical lymphadenopathy is aspirated. If this value is twice or highr than that of a simultaneous serum determination, it is suggestive of metastases even with a negative cytology.