In their excellent and comprehensive review on thyrotoxicosis (1), Vaidya and Pearce do not mention an often difficult to recognise cause of thyrotoxicosis; namely thyrotoxicosis factitia due to surreptitious thyroid hormone ingestion.
Patients present with hyperthyroidism and may be mistaken for Graves’ disease, if TSH receptor positive (2), or thyroiditis because of absent uptake on a thyroid radionuclide uptake scan due to suppression of thyroid function by exogenous thyroid hormones. Suppression of thyroid function also decreases thyroglobulin secretion. Serum thyroglobulin is therefore undetectable in thyrotoxicosis factitia which differentiates it from other causes of hyperthyroidism, in which serum thyroglobulin is elevated (3). Caution, however, should be exercised in interpreting thyroglobulin results without thyroglobulin antibodies, since thyroglobulin antibodies commonly interfere in thyroglobulin immunoassays causing false positive and negative results (4) which may lead to clinical misdirection (2). In such cases, increased faecal thyroxine levels in thyrotoxicosis factitia may help differentiate it from other causes of hyperthyroidism (5).
1. Vaidya B, Pearce SH. Diagnosis and management of thyrotoxicosis. BMJ 2014;349:g5128 doi: 10.1136/bmj.g5128
2. Jahagirdar VR, Strouhal P, Holder G, Gama R, Singh BM. Thyrotoxicosis factitia masquerading as recurrent Graves’ disease: Endogenous antibody immunoassay interference, a pitfall for the unwary. Ann Clin Biochem. 2008; 45: 325-7
3. Mariotti S, Martino E, Cupini C. Low serum thyroglobulin as a clue to the diagnosis of thyrotoxicosis factitia. N Engl J Med 1982; 307:410-2.
4. Clark P, Franklyn J Can we interpret serum thyroglobulin results? Ann Clin Biochem. 2012;49:313-22.
5. Bouillon R, Verresen L, Staels, F Bex M, De Vos P, De Roo M. The measurement of fecal thyroxine in the diagnosis of thyrotoxicosis factitia. Thyroid. 1993; 3:101-3.
Competing interests: No competing interests