Case 1 was a 51-year-old man diagnosed with thyrotropin (TSH)-secreting pituitary tumor. The octreotide loading test showed suppression of TSH secretion. Treatment with lanreotide preoperatively at 90 mg/month resulted in normalization of thyroid function. Three months after treatment initiation, tumor shrinkage was observed, and pituitary tumor resection was performed through transsphenoidal surgery. Case 2 was a 47-year-old woman in whom the octreotide loading test showed suppressed TSH secretion. Treatment with lanreotide preoperatively at 90 mg/month resulted in normalization of thyroid function. After six months of treatment, tumor reduction was observed, and transsphenoidal surgery was performed. In both cases, lanreotide administration before TSH-secreting pituitary tumor resection achieved normalization of thyroid function and tumor shrinkage. Treatment with lanreotide seems effective in patients who show TSH secretion suppression in the octreotide loading test.