ThyrotropinomA: Diagnosing a Rare Cause of Hyperthyroidism

Case: The patient is a 40-year-old male who presented for evaluation of hyperthyroidism with symptoms including palpitations, increased bowel movements, anxiety, and worsening tremor. With a family history of Graves’ disease and an ultrasound showing a hyperemic thyroid, there was initial suspicion for Graves’ disease. Although a radioactive iodine thyroid scan showed diffuse uptake that was elevated, 21.4% at 4 hours and 33.1% at 24 hours, lab evaluation appeared inconsistent with Graves’ disease revealing: TSH 1.65 µU/mL (upper limit of normal [ULN] 4.5), free thyroxine (FT4) 1.99 ng/dL (ULN 1.17), free triiodothyronine 6.16 ng/dL (ULN 3.98), thyroid stimulating immunoglobulin 92% (ULN 122), and thyroid receptor antibodies less than 1.0 IU/L (ULN 1.75). Lab results were reproducible with elevated FT4 even by equilibrium dialysis at 3.9 ng/dL (ULN 2.4) and high-normal TSH with serial dilution that ruled out assay interreference. Given these findings, our focus turned to rare causes of hyperthyroidism including thyrotropinoma and thyroid hormone resistance (RTH). Unique to the diagnosis of thyrotropinoma is an elevated serum α subunit in 50-85% of cases (1). Therefore, we obtained an α subunit level which was 0.35 ng/mL (ULN 0.55) with a molar ratio of 2.2 (ULN 2.4). Since the α subunit level was normal, the patient obtained genetic testing for mutations in the thyroid hormone receptor β gene seen in 85% of RTH cases (1). However, no sequence variants were identified. Since initial lab and genetic analyses were undifferentiating, additional tests were obtained including an insulin-like growth-factor 1 level of 209 ng/mL (ULN 237) and prolactin level of 10.1 ng/mL (ULN 20) which can be elevated in 30% of mixed thyrotropinoma cases (1). The first evidence to suggest a thyrotropinoma was a mildly elevated sex-hormone binding globulin at 102 nmol/L (ULN 80). Further evaluation with pituitary magnetic resonance imaging showed a 6-milimeter lesion. Although the pituitary lesion is suggestive of a thyrotropinoma, it is not definitive, as they are present in 20% of RTH cases (1). Therefore, with increased suspicion of a thyrotropinoma, we pursued the more robust T3 suppression test which showed 56% suppression of TSH consistent with a thyrotropinoma. The patient had pituitary surgery with pathology confirming weak immunoreactivity for TSH. Post-operatively, his symptoms improved and free thyroid hormones normalized. Discussion: It is important to distinguish between thyrotropinoma and RTH as the treatment is different with 80% of thyrotropinoma cases achieving euthyroidism after surgery (1). In our case, the diagnosis was initially unclear thus it was important to broaden the lab and genetic evaluation considering the limitations of the studies. One such limitation is the α subunit may not be elevated in microadenomas as occurred in our case. 1. Beck-Peccoz et al. J Endocrinol Invest. 2019; 42:1401-6