What are the causes of hyperthyroidism due to pitutary reasons and how common they are?

Very rare. The pituitary produces a hormone called TSH which tell the thyroid gland to produce thyroid hormones. If you have a tumor in the pituitary that produces too much tsh, this will lead to pituitary induced hyperthyroidism. This is very rare, apprx 1 in a million. If your initial labs suspect you have pituitary hyperthyroidism, I suggest you repeat the test as it's probably a lab error.