A 21 year old Caucasian female with no known personal or family history of thyroid disease presented with several years history of anxiety and was found to have abnormal thyroid function tests. Results showed TSH: 1.8 mu/l (0.35.5), T4: 73 pmol/l (1022), T3 12.3 pmol/l (3.16.8). Past medical history included chronic anxiety with no regular medications or nutritional supplements. There was no history of recent iodinated contrast administration, illness or amiodarone use. Clinically she was euthyroid with no goitre.
Assay interference was excluded with dual platform analysis: DELFIA and CENTAUR, which showed good agreement with the local Roche assay with respect of the TSH, T4 and T3. The SHBG was normal which made a thyrotoxic state due to a TSH-secreting pituitary adenoma unlikely. Genetic analysis confirmed a diagnosis of thyroid hormone resistance. Family screening was discussed.
Thyroid hormone resistance is a rare but recognised cause of hyperthyroxinaemia with a non-suppressed TSH. Usually levels of free thyroxine (FT4) are mildly elevated. In our patient, the symptoms of anxiety and weight loss led to treatment with carbimazole due to concerns over the markedly elevated T4 levels. This case illustrates that thyroid hormone resistance should be considered in the differential diagnosis with a non-suppressed TSH and markedly elevated serum free thyroxine. The levels of hyperthyroxinaemia are unusual but should not detract from considering the diagnosis of Resistance to Thyroid Hormone Syndrome.