DISCORDANT THYROID FUNCTION TESTS

RTH SYNDROME OR TSH-OMA?

Abstract

INTRODUCTION
Resistance to thyroid hormone (RTH) syndrome and TSH secreting pituitary adenomas are important causes of discordant thyroid function tests (TFTs). Differentiating the two clinical entities requires thorough clinical history taking with a combination of laboratory tests and radiological imaging. We present a young lady with discordant TFT who has RTH syndrome and also a pituitary microadenoma.

CASE
A 25-year-old lady with no prior medical illness who underwent routine laboratory tests was found to have raised thyroid stimulating hormone (TSH) with raised free thyroxine (FT4) and normal triiodothyronine (FT3) levels. This pattern remained the same with repeated TFTs, including those that were done at a separate laboratory using a different analyser. She was not taking any supplements or traditional medications and there was no known family history of thyroid illness. She remained clinically euthyroid with no apparent goitre. Anti-TPO and anti- thyroglobulin antibodies were positive and MRI brain revealed a pituitary microadenoma. A TRH stimulation test showed exaggerated TSH response suggestive of resistance to thyroid hormone (RTH) syndrome.

CONCLUSION
In a patient with raised FT4 and inappropriately normal or raised TSH, once assay interference has been ruled out, it is important to differentiate a TSH secreting pituitary adenoma from RTH syndrome. Although our patient had a pituitary microadenoma, her lack of symptoms and TRH stimulation test findings were suggestive of RTH syndrome. A theoretical probability of developing thyrotroph adenomas due to longstanding increase in thyrotroph activity has been suggested with one reported case so far.