Pituitary function tests

The pituitary gland is an endocrine gland at the base of the brain, which controls several critical hormonal axes.

The anterior pituitary, mostly under the control of neuroendocrine cells in the hypothalamus, produces:¹

• Adrenocorticotrophic hormone (ACTH), which stimulates glucocorticoid (including cortisol), mineralocorticoid, and androgen production from the adrenal glands.

• Thyroid-stimulating hormone (TSH), which stimulates thyroid hormone production in the thyroid gland.

• Luteinizing hormone (LH) and follicular stimulating hormone (FSH), which regulate sex hormone production from the gonads and reproductive function.

• Growth hormone (GH), which has multiple effects on growth, carbohydrate, and lipid metabolism in a variety of tissues, and stimulates insulin-like growth factor 1 (IGF-1) from the liver.

• Prolactin, which primarily stimulates milk production in mammary glands.

The posterior pituitary releases:

• Arginine vasopressin, vasopressin, or antidiuretic hormone (ADH), which primarily promotes free water reabsorption in the kidneys.

• Oxytocin, which stimulates uterine contractions during childbirth, stimulates milk production, and has various behavioural, emotional, and metabolic effects.

Pituitary dysfunction is rare. It can take the forms of:

• Excessive pituitary hormone production, usually due to hormone-secreting pituitary tumours.²

  • These tumours may cause visual field defects, classically a bitemporal hemianopia that may initially be limited to the superior temporal quadrants.³

  • They may also be visible on dedicated imaging of the pituitary, such as MRI.

• Deficient pituitary hormone production (hypopituitarism), which can be partial or complete.

Some pituitary hormones can be easily measured directly, including from primary care, namely: TSH, prolactin, FSH, and LH. Others require more specialised tests, which are usually only available in secondary or tertiary care. These are performed if there is a strong suspicion of pituitary dysfunction based on the clinical picture, including the results of initial screening tests.

This article describes those specialised tests. The precise protocols for the tests and the defined normal ranges vary between laboratories. Examples of protocols can be found by following the links in the 'Further reading & references', below.

Anterior pituitary hormone deficiencies

See also the separate Hypopituitarism article.

Anterior pituitary function can be assessed with a simultaneous 'triple test' or 'combined test', which involves:⁴

• Administering insulin, and measuring growth hormone and cortisol release in response to hypoglycaemia.

• Administering gonadotrophin-releasing hormone (GnRH), and measuring the response in LH and FSH levels.

• Administering thyrotrophin-releasing hormone, and measuring the response in TSH levels.

This is sometimes performed where multiple pituitary hormone deficiencies are suspected, and a single test is desired.

Tests for specific anterior pituitary hormones are described below.

Thyroid stimulating hormone

See also the separate Thyroid function tests article.

• TSH is directly measured in standard thyroid function tests.

• In secondary (central) hypothyroidism, free T4 levels are low, and TSH levels are low or inappropriately normal.⁵ Rarely, TSH levels may be slightly increased, in some patients with central hypothyroidism due to a hypothalamic defect, where the TSH molecules produced do not have full biological activity.⁶

• The TRH stimulation test can further differentiate between secondary (pituitary) hypothyroidism and tertiary (hypothalamic) hypothyroidism, and is sometimes done as part of a specialist workup.⁶

• It may also be useful to evaluate for central hypothyroidism where there is a strong suspicion for hypopituitarism, despite normal T4 levels.⁷

Growth hormone

• Growth hormone levels are pulsatile and almost undetectable for most of the day and so growth hormone levels are measured by their response to stimulation.

• Low baseline measurements of insulin-like growth factor 1 (IGF-1), insulin-like growth factor 2 (IGF-2), and insulin-like growth factor-binding protein 3 (IGFBP-3) suggest growth hormone deficiency. IGF-1 is most commonly measured.

• There are a number of different stimulation tests but the gold standard is insulin-induced hypoglycaemia, which is also used for cortisol deficiency. Alternative drugs used for stimulation include glucagon, clonidine, arginine and propranolol.

• Insulin tolerance test (also called insulin stress test):
  

  • The insulin tolerance test is generally regarded as the best test to stimulate growth hormone and ACTH secretion as long as there are no contra-indications.

  • Intravenous glucose may be required if severe hypoglycaemia occurs, and an indwelling cannula should be inserted for the duration of the test.

  • Contra-indications include age over 60 years, coronary heart disease, epilepsy, untreated hypothyroidism, severe panhypopituitarism and hypoadrenalism (an Addisonian crisis may be precipitated if there is no adrenal reserve).

  • Procedure:
    

    • Blood is taken for growth hormone, cortisol and glucose and then fast-acting insulin is injected intravenously. Blood is taken again for growth hormone, cortisol and glucose after 30, 45, 60, 90 and 120 minutes.

    • The test cannot be interpreted unless hypoglycaemia (<2.2 mmol/L) is achieved. There must be at least two specimens following adequate hypoglycaemia.

  • Interpretation: an adequate growth hormone response occurs with an absolute response >20 mU/L (6 μg/L).

ACTH

See also the separate Adrenal insufficiency and Addison's disease article.

• Cortisol: Serum cortisol level is measured at around 9 am. If low, this may be due to hypoadrenalism or ACTH deficiency.

• Synacthen test:
  

  • Measures the adrenal response to ACTH.

  • Plasma ACTH should be measured at the same time as the ACTH stimulation, although this test is considered to be inaccurate. The ACTH plasma and ACTH stimulation test together can give a clearer picture, especially in the case of secondary adrenal insufficiency.

  • Patients taking oral steroids must omit them on the morning of the test but take them as soon as the test is completed. Oestrogen-containing medications should be stopped for six weeks prior to measuring serum cortisol.

  • Synacthen® (an ACTH analogue) is administered as an intravenous bolus with measurements of cortisol (and for 17-hydroxyprogesterone where the test is being performed for possible congenital adrenal hyperplasia) at 0 minutes, 30 minutes and 60 minutes.

  • This test has a high specificity but low sensitivity. Adrenal insufficiency is excluded by an incremental rise in cortisol of >200 nmol/L and a 30-minute value >600 nmol/L.

  • A low-dose 1-μg ACTH stimulation test can be used to increase the specificity of the high-dose test. The low dose of ACTH does not stimulate cortisol production in an unprimed adrenal gland.

  • Chronic ACTH deficiency from pituitary or hypothalamic dysfunction leads to adrenal atrophy, and therefore the cortisol response to the Synacthen test may also be reduced or absent, although in some cases (particularly if the ACTH deficiency is recent) it may be normal.¹ Concomitant measurement of serum ACTH may help to differentiate - a positive Synacthen test (little or no cortisol response) with high serum ACTH levels suggests primary adrenal insufficiency.

• Insulin tolerance/stress test (see 'Growth hormone', above):
  

  • If there is adequate hypoglycaemia and the patient is not hypothyroid then the cortisol response is a good test of ACTH/adrenal reserve.

  • An adequate cortisol response is defined as a rise to >550 nmol/L. However, 5-15% of normal subjects will show a suboptimal response as defined by cortisol <550 nmol/L.

  • Glucagon can be used as an alternative to insulin for stimulating production of ACTH. Glucagon is particularly useful when insulin-induced hypoglycaemia is contra-indicated

• Metyrapone test: metyrapone blocks adrenal hormone production and normal individuals respond by producing large amounts of ACTH. Lack of response indicates pituitary disease affecting ACTH production.⁸

• Single measurement of plasma dehydroepiandrosterone sulfate levels have been shown to be at least a reasonable screening test for ACTH deficiency, with good sensitivity and specificity.⁹ Normal levels are age-dependent and the laboratory must have age-matched control normal levels available.

Gonadotrophin deficiency¹⁰¹¹

• Gonadotrophin deficiency is difficult to assess in girls prior to puberty or in boys after age 3-6 months and before puberty.

• Follicle-stimulating hormone (FSH), luteinising hormone (LH) and either estradiol or testosterone (as appropriate for sex): hormone deficiency can be difficult to assess and hormone levels can be particularly affected by stage of puberty and menstrual cycle. In gonadotrophin deficiency, however, basal levels are often enough for diagnosis, without stimulation testing as below.

• GnRH testing:
  

  • The main use is to confirm precocious puberty rather than deficiency.

  • Samples are obtained to measure LH and FSH levels at baseline and at 20 minutes and 60 minutes after giving a short-acting GnRH analogue. Samples for testosterone (males) and estradiol (females) are also taken at the baseline.

  • In healthy pubertal children, both LH and FSH should at least double at 20 minutes and LH response is usually greater. In normal prepubertal children and patients with complete gonadotrophin deficiency, little to no response of either LH or FSH occurs.

  • An intermediate response does not distinguish prepubertal gonadotrophin deficiency from simple delayed puberty.

• Kisspeptin, hCG, and clomiphene stimulation tests are sometimes used as alternatives.¹¹¹

Anterior pituitary hormone hypersecretion

Prolactin

See also the separate Hyperprolactinaemia and prolactinoma article.

• Levels should be taken on at least two occasions and after relaxation if elevation is suspected. A single prolactin measurement may be sufficient if the value is >200 μg/L.

• Circulating prolactin levels are lowest at midday, with a modest increase occurring during the afternoon. Prolactin levels increase during rapid eye movement (REM) sleep, in response to stress and following meals.

ACTH

See the separate Cushing's syndrome article.

• After ruling out exogenous causes, the dexamethasone suppression test (DST), late-night salivary cortisol and 24-hour urinary-free cortisol are used to diagnose hypercortisolism.¹² They all have limitations related to their sensitivity and specificity. In addition, cortisol secretion fluctuates, and thus, assessments are subject to variability over time. Because no single test is perfect, the performance of more than one test, either sequentially or simultaneously, is a common practice.

Growth hormone

See also the separate Acromegaly article.

• A single measurement of growth hormone is inadequate because growth hormone is secreted in a pulsatile manner during deep sleep. A series is taken.

• Serum IGF-I: measurement of serum IGF-I concentration is a sensitive screening test for acromegaly. Significantly higher IGF-I levels occur during puberty than those during adulthood. For accurate control comparison, the IGF-I level must be compared with that of control subjects who are matched for age, gender and Tanner's stage.

• Serum IGFBP-3: increased IGFBP-3 level has been reported as a sensitive marker of growth hormone hypersecretion.

• Inability to suppress serum growth hormone levels during an oral glucose tolerance test (OGTT): the single best laboratory criterion for diagnosing growth hormone excess is failure to suppress serum growth hormone levels to >5 ng/dL within three hours after an oral glucose challenge.

Gonadotrophins

See also the separate Precocious puberty article.

• A random LH is a useful initial test for precocious puberty. A random FSH will not distinguish prepuberty from puberty. Low or prepubertal levels with high sex steroid levels are found in gonadotrophin-independent precocious puberty.

• GnRH stimulation test: as in the gonadotrophin deficiency section above, LH and FSH levels are measured sequentially after GnRH stimulation. The test is useful in the assessment of precocious puberty.¹³

• When GnRH is unavailable the synthetic analogue, leuprorelin acetate is a suitable alternative and can accurately predict pubertal progression.¹⁴

Posterior pituitary

The posterior pituitary produces antidiuretic hormone (ADH) and oxytocin, which stimulates uterine contraction during birth and ejection of milk during lactation. Patients with suspected diabetes insipidus are usually assessed by a water deprivation test. Alternatively, a non-osmotic stimulus such as hypoglycaemia may be used to stimulate ADH secretion.

• Initial tests: plasma glucose, U&Es; plasma and urine osmolality.

• Water deprivation test:
  

  • The water deprivation test is used in the differential diagnosis of polyuria, differentiating between cranial diabetes insipidus (CDI), nephrogenic diabetes insipidus (NDI) and primary polydipsia (compulsive water drinking).

  • Anterior pituitary hormone deficiency: renders results meaningless as, in particular, steroid and thyroxine deficiencies impair excretion of a free water load.

  • For patients with true CDI or NDI, there is a risk of excessive dehydration.

  • Stage 1 (exclusion of primary polydipsia) - 8.30-16.30 hours:
    

    • No fluids are allowed. the patient is weighed at the beginning of the test and at hourly intervals. This stage of the test should be stopped if there is more than 3% weight loss (positive test).

    • Urine is passed and discarded at the beginning of the test and then passed hourly and tested for hourly urine volume and osmolality. Blood is taken for osmolality at intervals during the test.

  • Stage 2 (differential diagnosis CDI from NDI) - 16.30-20.30 hours: the patient may eat and drink freely. DDAVP® is given either intranasally or intramuscularly and hourly urine volumes and osmolality measurements are continued.

• For interpretation of results and other investigations for CDI and NDI, see the separate Diabetes insipidus article.