65 yr female presented with headaches, heat intolerance and loose stools. She had cerebrovascular accident, osteoporosis and vitiligo. A 24 hr ECG revealed atrial flutter with heart rate upto 117 beats /minute. Her medications include warfarin and bisoprolol.
A Thyroid function test (TFT) revealed Free T4 (Ft4): 32 pmol/L (9-19) Free T3: 7.97 pmol/L (3.66.5), TSH: 5.7 mU/L (0.354.94). Other anterior pituitary function tests were normal.
In view of toxic symptoms an MR scan of the pituitary was done. It showed a right sided microadenoma suggesting the possibility of a micro-TSHoma.
Blood tests revealed alpha subunit levels were normal but sex hormone binding globulin (SHBG) was raised.
Subsequently the thyroid hormone receptor beta genetic screening test was positive confirming the presence of Syndrome of Resistance to thyroid hormone (RTH).
To rule out a co-incident micro-TSHoma a methionine PET scan was done (protocol by Dr Gurnell, Cambridge) confirming a metabolically active lesion. A TRH test revealed a brisk TSH response. An Octreotide suppression test (OST) followed by a trial of Somatostatin analogue (SSA) was then done to differentiate between a TSHoma and pituitary incidentaloma.
The OST test showed a baseline TSH of 4.7 mU/L (0.355.5) and 5 hrs of 2.6 mU/L. A depot SSA trial did not show normalisation of thyroid function test.
The one year repeat Pituitary MRI showed unchanged appearances along with lateralised PET scan uptake.
Discussion: Both TSHoma as well as RTH are rare syndromes. Though the genetic test was positive for RTH some of the investigations including SHBG, MRI pituitary and positive PET were suggestive of coexistent micro-TSHoma. A six month trial with SSA failed to normalise TFT. This suggested a pituitary incidentaloma rather than a micro-TSHoma, avoiding surgery. Our case demonstrates the challenges faced in diagnosing and differentiating a micro-TSHoma from a pituitary incidentaloma when an RTH syndrome is present.