Negative feedback — the idea behind every endocrine gland

Most hormones are controlled by negative feedback: the effect produced by the hormone switches off (or reduces) the secretion of that same hormone, which prevents over- and under-secretion.

• Thyroid: hypothalamus → pituitary releases TSH → thyroid releases thyroxine → rising thyroxine in blood inhibits the pituitary (TSH falls) → thyroxine falls. When thyroxine is low (iodine deficiency) the inhibition is lost, TSH keeps rising, and the gland enlarges: simple goitre.
• Blood calcium: low Ca²⁺ → parathormone ↑ (calcitonin ↓) → Ca²⁺ rises → parathormone falls. High Ca²⁺ → calcitonin ↑ → Ca²⁺ falls.
• Blood glucose: high → insulin ↑, glucagon ↓; low → glucagon ↑, insulin ↓.
• Sex hormones: oestrogen and progesterone from the ovary inhibit FSH and LH (this is why the contraceptive pill stops ovulation).