A series of biochemical parameters are useful for the diagnosis and follow-up of differentiated thyroid carcinomas. The measurement of serum thyroglobulin (Tg) is considered for the post-surgical/radioiodine follow-up of papillary/follicular carcinomas. Other than in basal conditions, the importance of Tg levels during TSH stimulation is underlined, either by discontinuation of L-T4 therapy or by recombinant human TSH test. The finding of undetectable Tg levels during TSH stimulation is highly correlated with clinical remission; otherwise, peak Tg levels > 1-2 ng/ml can be suggestive of recurrence/persistence of the disease. The accuracy of Tg measurements for the detection of metastases seems to be higher when compared to 131-I whole-body scan. The evaluation of basal serum calcitonin levels is recommended for the screening of medullary thyroid carcinoma (MTC). High basal levels suggest the presence of a tumor but a calcitonin increase can be observed also in parafollicular C cell hyperplasia (CCH) and other extra-thyroidal conditions. The pentagastrin test can by pass this obstacle as the calcitonin response seems to be typical of pathological thyroid C cells. The cut-off value of calcitonin response between patients with MTC and CCH remains to be established; the latter condition, however, being considered by some authors as pre-cancerous. After thyroid surgery the measurement of calcitonin constitutes a valid tool for the detection of cure and/or recurrence of the disease. The screening by means of RET oncogene analysis is also described for patients with MTC with Multiple Endocrine Neoplasia (MEN) type 2 syndrome.