Thyrotoxicosis factitia: role of thyroglobulin

We wish to clarify an important point regarding the differential diagnosis of thyrotoxicosis with reduced uptake on a thyroid radionucleotide uptake scan, which includes thyroiditis, ectopic thyroxine production and thyrotoxicosis factitia due to thyroid hormone ingestion.

A thyroid Tc99 uptake scan will not, as stated (Ann Clin Biochem 2008;45:215–217), ¹ differentiate between thyrotoxicosis factitia due to thyroid hormone ingestion and thyroiditis since thyroidal radionucelotide uptake is low or absent in both conditions. The destructive nature of thyroiditis and suppression of endogenous thyroid function by exogenous thyroid hormones, respectively, reduce thyroidal radionucleotide uptake.

Suppression of endogenous thyroid function by exogenous thyroid hormones also decreases thyroglobulin secretion. Serum thyroglobulin is, therefore, undetectable following excessive thyroid hormone ingestion, thus differentiating thyrotoxicosis factitia from other causes of hyperthyroidism, including thyroiditis, in which serum thyroglobulin is elevated. ² Increased faecal thyroxine concentrations in thyrotoxicosis factitia may also differentiate it from other causes of hyperthyroidism. ³