Pituitary function tests

Tests of the pituitary gland and pituitary function include:

• Hormone levels: pituitary hormones are released in pulses and levels fluctuate during the day. A single blood test is usually insufficient to assess a pituitary hormone deficiency properly.

• Hormone stimulation 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.

• Visual field check: pituitary tumours may cause visual field defects.

• MRI or CT scan of the pituitary and surrounding structures.

Anterior pituitary hormone deficiencies

See also the separate Hypopituitarism article.

A 'complete anterior pituitary function test' includes:

• Gonadotrophin-releasing hormone (GnRH) stimulation. GnRH is also known as luteinising-hormone-releasing hormone (LHRH).

• Thyrotrophin-releasing hormone (TRH) stimulation

• Insulin tolerance test (or alternative method of stimulating growth hormone and adrenocorticotropic hormone (ACTH) secretion).

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, ACTH deficiency is likely unless the person is taking steroid medication. After ruling out exogenous causes, the dexamethasone suppression test (DST), late-night salivary cortisol and 24-hour urinary-free cortisol are used to diagnose hypercortisolism¹. 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.

• 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. In ACTH deficiency the response to the short test may be normal or reduced.

  • 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.

• 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.

• 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 specificity². Normal levels are age-dependent and the laboratory must have age-matched control normal levels available.

Gonadotrophin deficiency

• 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.

Thyroid-stimulating hormone (TSH)

See also the separate Thyroid Function Tests article.

• Thyrotrophin-releasing hormone (TRH) stimulation test:
  

  • A TRH test may be indicated if secondary hypothyroidism is suspected.

  • A small amount of TRH is given intravenously and then blood is taken for levels of TSH at several subsequent time points.

  • Patients with normal function of the hypothalamic-pituitary axis respond by increasing the levels of TSH following TRH injection. Patients with compromised HPA function may exhibit a delayed, blunted or absent response to TRH administration.

  • TRH stimulation test may be useful in the diagnosis of central hypothyroidism, especially if free T4 and/or TSH are low-normal³⁴.

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.

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 puberty⁵.

• When GnRH is unavailable the synthetic analogue, leuprorelin acetate is a suitable alternative and can accurately predict pubertal progression⁶.

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.