Endocrine Abstracts

Introduction: Normal or elevated TSH level in the presence of elevated T₄ is defined as ‘inappropriate TSH syndrome’. Two main clinical conditions that can lead that syndrome are TSH-secreting adenoma (TSHoma) and resistance to thyroid hormone (RTH). Making the correct diagnosis is crucial in order to decide the most appropriate treatment option. Herein we presented clinical and laboratory data of seven patients who were hospitalised for the differential diagnosis of the two clinical entities.

Method: Our database was reviewed for the patients diagnosed with inappropriate TSH syndrome at our hospital between 2010 and 2014. After exclusion of the other rare causes of inappropriate TSH syndrome, seven patients who were hospitalised for the differential diagnosis of TSHoma and RTH were included in this report.

Result: Age of the patients was changing between 20 and 52 years. Clinically two patients were asymptomatic, three had tachycardia and weight loss and one had goitre. Final diagnosis was RTH in four patients, TSHoma in two and unequivocal in one. Two patients diagnosed with TSHoma were operated and had positive staining with TSH. Both of the TSHoma cases had macroadenoma on pituitary MRI and visual field defect while two of four patients with RTH had microadenoma. Alpha-subunit/TSH molar ratio was above 1 in all patients diagnosed with TSHoma while it exceeded 1 in two patients with the final diagnosis of RTH. TRH stimulation test revealed a blunted response in all patients with TSHoma and a positive response (increase >100%) was observed in all with THR. TSH was suppressed after T₃ suppression test in all patients with the final diagnosis of RTH.

Conclusion: Differential diagnosis of RTH and TSHoma can be a clinical challenge and requires complex hormonal tests and imaging methods. Since incidental pituitary tumours are not rare, presence of an adenoma should not rule out diagnosis of RTH.