Tests of 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 'Internet and further reading' section at the end of this article.
- 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 separate article Hypopituitarism. A 'complete anterior pituitary function test' includes gonadotrophin-releasing hormone (GnRH) stimulation, thyrotrophin-releasing hormone (TRH) stimulation and the insulin tolerance test (or alternative method of stimulating growth hormone and adrenocorticotropic hormone (ACTH) secretion). GnRH is also known as luteinising-hormone-releasing hormone (LHRH).
- Growth hormone levels are 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, insulin-like growth factor 2, and insulin-like growth factor-binding protein 3 suggest growth hormone deficiency.
- There are a number of different stimulation tests but the gold standard is insulin-induced hypoglycaemia, which is also used for cortisol deficiency. Using an arginine infusion for stimulation is probably as effective and other drugs used include glucagon, clonidine, and propranolol.
- Insulin stress test (also called insulin tolerance test):
- The insulin stress 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, ischaemic heart disease, epilepsy, severe panhypopituitarism and hypoadrenalism (an Addisonian crisis may be precipitated if there is no adrenal reserve).
- 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 (7 μg/L).
See also separate article Adrenal Insufficiency and Addison's Disease.
- Cortisol: measurement of salivary cortisol can provide important information about hypothalamic-pituitary-adrenal (HPA) axis activity under normal conditions and in response to stress. However, many variables affect the cortisol level, such as the time the sample was taken and physical and psychological activity.
- Synacthen test:
- Measures the adrenal response to adrenocorticotropic hormone (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. A peak cortisol level below 18 μg/dL is suggestive of cortisol deficiency either because of ACTH deficiency or because of primary adrenal disease.
- 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 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 greater than 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 has recently been shown to be at least a reasonable screening test for ACTH deficiency, with good sensitivity and specificity. Normal levels are age-dependent and the laboratory must have age-matched control normal levels available.
- 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.
- Gonadotrophin-releasing hormone (GNRH) testing:
- Samples are obtained to measure LH and FSH levels at baseline and at 2 hours, 4 hours, 6 hours, and 24 hours after giving a short-acting GnRH analogue. For males, baseline and 24-hour testosterone levels may also be helpful.
- In healthy pubertal children, the LH response exceeds the FSH response. In prepubertal children, the FSH response exceeds the LH response. In patients with complete gonadotrophin deficiency, little to no response of either LH or FSH occurs.
- Gonadotrophin stimulation test: an increase in testosterone levels in males after 24 hours suggests an intact hypothalamic-pituitary-gonadal axis. An intermediate response does not distinguish prepubertal gonadotrophin deficiency from simple delayed puberty.
Thyroid-stimulating hormone (TSH)
See also separate article Thyroid Function Tests.
- 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 is useful in the diagnosis of central hypothyroidism, especially if free T4 and/or TSH is low-normal.
- Normal baseline serum prolactin levels are less than 20 μg/L in non-gravid women and are usually less than 10 μg/L in men. 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.
- Prolactin deficiency can be confirmed by administering TRH or an anti-dopaminergic agent (eg metoclopramide) and measuring prolactin levels. Failure of the prolactin level to rise is diagnostic of prolactin deficiency.
Anterior pituitary hormone hypersecretion
See also separate article Hyperprolactinaemia.
- A single prolactin measurement may be sufficient if the value is greater than 200 μg/L.
- TRH stimulation test:
- In healthy patients, TRH results in a brisk rise in serum prolactin in 15-30 minutes, with peak values at least twice the baseline value.
- Patients with prolactin-secreting tumours usually show little or no prolactin increase in response to TRH. Patients with elevated serum prolactin from other causes usually show a more normal response, with a rise in prolactin of at least 100% following TRH administration.
See separate article Cushing's Syndrome.
See also separate article Acromegaly.
- A single measurement of growth hormone is inadequate because growth hormone is secreted in a pulsatile manner during deep sleep.
- Serum insulin-like growth factor-I (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 insulin-like growth factor-binding protein-3 (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 less than 5 ng/dL within 3 hours after an oral glucose challenge.
See also separate article Precocious Puberty.
- 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: LH and FSH levels are measured sequentially after GnRH stimulation. The test is useful in the assessment of precocious puberty. There is a flat response in gonadotrophin-independent precocious puberty.
- Leuprorelin acetate stimulation testing is an alternative and can accurately predict pubertal progression..
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 (see also separate article Diabetes Insipidus):
- 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 hrs:
- 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 hrs: 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 links for the separate articles at the top of this section.
Further reading & references
- Pituitary Disease Factfile for General Practitioners, The Pituitary Foundation
- Biochemical Investigations in Laboratory Medicine, Investigation protocols
- Diagnostic tests, EndocrineSurgeon.co.uk
- Endobible, Investigation protocols
- Hoffman RP; Panhypopituitarism, Medscape, Oct 2009
- Jessop DS, Turner-Cobb JM; Measurement and meaning of salivary cortisol: a focus on health and disease in Stress. 2008;11(1):1-14. Epub 2007 Jul 16.
- Fischli S, Jenni S, Allemann S, et al; Dehydroepiandrosterone sulfate in the assessment of the J Clin Endocrinol Metab. 2008 Feb;93(2):539-42. Epub 2007 Nov 6.
- Atmaca H, Tanriverdi F, Gokce C, et al; Do we still need the TRH stimulation test? Thyroid. 2007 Jun;17(6):529-33.
- Ferry RJ et al; Hyperpituitarism, Medscape, Aug 2010
- Kandemir N, Demirbilek H, Ozon ZA, et al; GnRH Stimulation Test in Precocious Puberty: Single Sample is Adequate for J Clin Res Pediatr Endocrinol. 2011 Mar;3(1):12-7. Epub 2011 Feb 23.
- Ng SM, Kumar Y, Cody D, et al; The gonadotrophins response to GnRH test is not a predictor of neurological lesion in girls with central precocious puberty.; J Pediatr Endocrinol Metab. 2005 Sep;18(9):849-52.
- Sathasivam A, Garibaldi L, Shapiro S, et al; Leuprolide stimulation testing for the evaluation of early female sexual Clin Endocrinol (Oxf). 2010 Sep;73(3):375-81. Epub 2010 Feb 23.
|Original Author: Dr Colin Tidy||Current Version: Dr Colin Tidy|
|Last Checked: 22/06/2011||Document ID: 13580 Version: 1||© EMIS|
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