TSH producing pituitary tumor: biochemical diagnosis and long-term medical management with octreotide

A 50 year old man with hyperthyroidism secondary to inappropriate secretion of TSH is described. On presentation T3 (42.1 nmol/L), T4 (265 nmol/L) and TSH (17.9 mU/L) were all markedly elevated. A diagnosis of a TSH-secreting pituitary tumor was suspected on the basis of a blunted TSH response to TRH and the absence of suppression of TSH by T3 or bromocriptine, but TSH/alpha subunit molar ratios were uncharacteristically less than 1. Nevertheless, the presence of a tumor was confirmed by computed tomography which demonstrated a 15 mm pituitary macroadenoma. The patient was treated with octreotide which resulted in normalisation of thyroid hormone levels. The duration of action of a single 100 micrograms injection of octreotide was at least 56 hours. The suppression of thyroid hormone levels was similar regardless of the treatment regimen with octreotide (100 micrograms tid, 250 micrograms bid, 100 micrograms bid and continuous subcutaneous infusion (CSI] and no escape was observed during a 16 month treatment period. TSH alpha subunit concentrations were also suppressed during long-term treatment with octreotide (3.3 micrograms/L falling to 1.1 micrograms/L), although no acute changes were noted after administration of single dose octreotide 100 micrograms. Three times the octreotide therapy was discontinued. The incremental rise in TSH and the maximum level of TSH achieved was less on each subsequent occasion, suggesting a suppressive effect of octreotide on the tumor itself. Despite suppression of TSH with octreotide over a 13 month period the pituitary tumor showed no shrinkage on repeat MRI scanning. In conclusion, this patient demonstrates that the differential diagnosis of inappropriate TSH secretion based only on biochemical test may be unreliable.(ABSTRACT TRUNCATED AT 250 WORDS)