Neuroendocrinology II

The pituitary gland is found behind the sphenoid sinus. Surgical removal of the pituitary gland (hypophysectomy) is frequently done trans-sphenoidally. The sella turcica, aka the hypophyseal fossa, is the cavity within the skull where the pituitary gland sits but it is not a sinus.

Vasopressin and oxytocin are released from the posterior pituitary gland but synthesised in the hypothalamus. Prolactin is released from the anterior pituitary gland but its production is inhibited by dopamine, which is secreted by the hypothalamus (sometimes referred to as the tubero-hypophyseal dopamine pathway).

The hypothalamus sends hormones to the anterior pituitary gland via a vein that runs through the pituitary stalk. A portal system is where venous blood flows from one capillary bed to a second capillary bed.

GHRH stimulates the production of hGH (human growth hormone). The body needs glucose to grow therefore hGH stimulates glucose secretion. ACTH stimulates corticosteroid release, one of the effects of which is an increase in blood glucose; in fact, corticosteroid therapy can increase blood glucose so is an important side effect to look out for, especially in diabetics. TSH would stimulate the production of thyroid hormones that increase metabolic activity in cells and therefore draw more glucose in from the blood stream; in fact hypothyroidism can cause diabetes in some patients. Insulin is released in response to high blood glucose, and ADH has no role in glucose regulation at all.

Risperidone is a classical antipsychotic and inhibits the effects of dopamine. Dopamine inhibits prolactin secretion, therefore the patient will have higher circulating levels of prolactin which has caused galactorrhoea.

These are symptoms of Cushing’s syndome which is caused by an increase of cortisol from the adrenal cortex. Cortisol secretion is regulated by ACTH (adrenocorticotrophic hormone). Therefore it is this hormone that is most likely to be raised. A raised TSH (and low T4) might be expected if the patient had hypothyroidism, but this would mainly account for their weight gain, not their other symptoms.

This patient has acromegaly secondary to a pituitary adenoma. A change in shoe or glove size is a characteristic symptom of this syndrome. It is caused by human growth hormone (hGH). As this is a pituitary problem, a rise in GHRH would not be responsible for this patient’s presentation, even though it stimulates hGH secretion.