Background: Factitious T4-thyrotoxicosis accounts for approximately 0.3% of all cases of thyrotoxicosis. Although recognised as a cause of hyperthyroidism, there are no recent peer-reviewed published cases of thyrotoxicosis secondary to chronic ingestion of tri-iodothyronine.
Case Report: A 23-year-old lady presented with hyperthyroidism. She had a history of PCOS and reported diffi culties with weight control. On examination, she had a soft, non-tender thyroid gland just palpable. Blood results were consistent with T3-thyrotoxicosis, but fT4 concentration was suppressed. Four days later, she was hospitalised with worsening symptoms of thyrotoxicosis. Within 24h of admission her symptoms settled and fT3 fell into the reference range. TFTs repeated 1 week post-discharge were again consistent with T3-thyrotoxicosis. It was then discovered that her mother was prescribed tri-iodothyronine for hypothyroidism. When questioned, the patient denied taking her mother’s medication.
Conclusions: Factitious T3-thyrotoxicosis should be considered in patients with intermittent hyperthyroidism where fT4 is suppressed and thyroid examination is normal. Factitious T3-thyrotoxicosis is much less common than factitious T4-thyrotoxicosis, as tri-iodothyronine is generally less available than levo-thyroxine. Increased public internet availability is likely to be associated with future increased incidence of factitious T3-thyrotoxicosis and therefore it is important to highlight the clinical features of this condition.

Keywords: Factitious thyrotoxicosis, hyperthyroidism, tri-iodothyronine