After thyroidectomy and 131I ablation for differentiated thyroid cancer (DTC), serum thyroglobulin (Tg) became a sensitive marker of residual disease. It is not uncommon to find patients at follow-up with persistent serum Tg levels and no other clinical or imaging evidence for the disease. The vast majority of these patients, most probably, have occult foci of disease, often in minute cervical lymph nodes. A review of the literature including papers published on PubMed/Medline until June 2010 was made. In this study we speculated that a minority of patients who had undergone surgery for differentiated thyroid cancer might have benign sources of Tg secretion at follow-up. These sources may be foci of radio-resistant ectopic thyroid tissue or a thyroid stimulating hormone-stimulated thymus.