What is the relation between the thyroid and the hypophysis?
The hypophysis secretes TSH, thyroid- stimulating hormone. This hormone hastens the secretion of thyroid hormones (triiodothyronine and thyroxine, or T3 and T4). When the plasma concentration of thyroid hormones is high this information is detected by the hypothalamus and by the hypophysis 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 example, in case of tissue destruction) or hypersecretion (in case, e,g., of excessive cell proliferation or cancer) can change the functioning of the thyroid gland completely.
