Factitious thyrotoxicosis secondary to Levo-thyroxine accounts for approximately 0.3% of all cases of hyperthyroidism. Although recognised as a cause of thyrotoxicosis, there are no peer-reviewed published cases of thyrotoxicosis secondary to ingestion of Tri-iodothyronine.
A 23-year old lady presented to endocrine clinic with a 6-month history of palpitations, heat intolerance and tremor. She had a history of PCOS and reported difficulties with weight control. On examination, she was overweight (BMI 27.4), clinically thyrotoxic and a soft thyroid gland was just palpable. Blood results were consistent with T3-thyrotoxicosis: TSH <0.01 mu/L (0.44.5) and fT3 24.6 pmol/L (2.56.5), although fT4 was low 9.7 pmol/L (10.023.0). Carbimazole had caused a rash, so propylthiouracil 150 mg bd was commenced.
4 days later she was hospitalised with worsening palpitations and tachycardia of 140 bpm. Propylthiouracil was continued and verapamil commenced (previous intolerance to beta-blockers). Within 24 h of admission her symptoms settled and fT3 fell from 8.3 pmol/L to 4.0 pmol/L.
TFTs repeated 1 week post-discharge were again consistent with T3-thyrotoxicosis (TSH <0.01 mu/L, fT3 13.1 pmol/L, fT4 9.0 pmol/L). Thyroid antibodies were negative. Heterophilic antibodies and PEG precipitation for antibody interference in TSH, fT4 and fT3 assays were all negative. It was then discovered that her mother was taking Tri-iodothyronine for hypothyroidism. When questioned, the patient denied taking her mothers medication.
Further visits to endocrine clinic have been associated with intermittent T3-thyrotoxicosis. At times of fT3 elevation, fT4 has always been suppressed. However, the patient continues to deny ingestion of Tri-iodothyronine.
Factitious T3 thyrotoxicosis should be considered in patients with intermittent hyperthyroidism where thyroid examination and thyroid antibodies are normal, particularly when fT4 is low-normal or suppressed. Factitious T3-thyrotoxicosis is much less common than factitious T4-thyrotoxicosis. This is probably because Tri-iodothyronine is rarely prescribed in the UK and is more expensive than Levo-thyroxine (internet search: Tri-iodothyronine 40Euros for 100×20 μg tablets, Levo-thyroxine 3Euros for 100×100 μg tablets).