The Pituitary And Why It Sits Above Everything

The pituitary is about the size of a pea and sits beneath the base of the brain. Its position in the hierarchy is what makes it disproportionately important.

It is the junction between brain and body chemistry

The hypothalamus monitors the internal state and the environment, then sends chemical instructions a very short distance to the pituitary through a dedicated blood supply.

The pituitary converts those instructions into hormones released into the general circulation, which then act on the thyroid, adrenal glands, ovaries or testes.

This makes it the point where neural information becomes endocrine command, and it is why states such as stress, starvation and illness affect hormone systems throughout the body.

Two lobes with different arrangements

The anterior lobe is glandular tissue that manufactures its own hormones under hypothalamic instruction, including those governing the thyroid, adrenal cortex and reproductive glands.

The posterior lobe is not really a gland but nerve endings extending down from the hypothalamus, releasing hormones made in the brain itself.

Those posterior hormones govern water retention by the kidney and uterine contraction and milk release, which is why the two lobes fail in quite different ways.

Feedback loops keep the outputs bounded

Each axis is regulated by negative feedback. The hormone produced by the target gland travels back and suppresses both the pituitary and the hypothalamus.

This is why measuring the pituitary hormone alongside the target gland's hormone is so informative. The pair together indicate where in the chain a problem sits.

A high pituitary signal with a low gland output points to the gland; a low signal with low output points upstream to the pituitary or hypothalamus.

Its location creates its own problems

The gland sits in a bony hollow immediately below the crossing of the optic nerves. Anything enlarging upwards presses on that crossing.

The result is a characteristic loss of the outer visual fields, which develops slowly enough that people often only notice it during an eye test or after an accident.

Pituitary tumours are usually benign, and many are found incidentally on imaging done for other reasons, which is why not all of them require treatment.

Why symptoms are so easily missed

Underactivity produces fatigue, low mood, loss of libido, cold intolerance and irregular periods — a set of complaints with many more common explanations.

Because several axes can be affected together or in sequence, the picture accumulates gradually and rarely presents as one obvious problem.

Anyone with a combination of unexplained fatigue, hormonal symptoms and visual changes should have that combination assessed rather than each part addressed separately.