Low serum thyroglobulin as a clue to the diagnosis of thyrotoxicosis factitia

SURREPTITIOUS ingestion of thyroid hormones may be responsible for thyrotoxicosis (thyrotoxicosis factitia). Identification of this condition often presents a difficult diagnostic challenge, since thyrotoxicosis factitia may be confused with other types of thyrotoxicosis associated with low radioiodine uptake.1 , 2 Serum thyroglobulin levels are elevated in many varieties of hyperthyroidism. Recently, some authors have suggested that low rather than elevated concentrations of circulating thyroglobulin in hyperthyroid patients may indicate the presence of thyrotoxicosis factitia. We have evaluated serum thyroglobulin levels in six patients with thyrotoxicosis factitia. In all cases serum thyroglobulin was undetectable by a sensitive immunoradiometric assay. Serial measurements in two patients with intermittent access to thyroid tablets showed normal and undetectable serum thyroglobulin levels associated with normal and increased serum thyroid hormone concentrations, respectively. Thus, serum thyroglobulin measurement can be used to identify patients with thyrotoxicosis factitia.