Q. What is the relation between the hypophysis and the thyroid?
The hypophysis secretes TSH (thyroid- stimulating hormone). This hormone hastens the secretion of thyroid hormones (thyroxine and triiodothyronine, or T4and T3).
When the plasma concentration of thyroid hormones is high this information is detected by the hypophysis and by the hypothalamus and this gland reduces the TSH secretion. When the thyroid hormone levels are low the TSH secretion increases it is thus a negative feedback.
Injuries of the hypophysis that cause TSH hyposecretion for instance in case of tissue destruction or hypersecretion in case for example, cancer or excessive cell proliferation, be able to change the functioning of the thyroid gland completely.