Optimal Use of Laboratory Tests in Pediatric Endocrine Practice

Transcription

1 Optimal Use of Laboratory Tests in Pediatric Endocrine Practice Michael J. Bourgeois Department of Pediatrics, Texas Tech University School of Medicine, Lubbock, Texas Abstract. Optimal use of the laboratory in the practice of pediatric endocrinology begins with careful consideration of the patient's history and physical findings. After a review of the differential diagnosis, an orderly plan of laboratory investigation can be devised. This paper outlines some of the more common laboratory tests used to evaluate some common pediatric endocrine disorders. The intent is to provide a concise, logical approach to choose the most efficient approach. [Indian J Pediatr 2000; 67 (3) : ] Key words : Pediatric endocrinology; Laboratory tests; Diagnosis. The use and interpretation of laboratory data in the practice of pediatric endocrinology must be done in the context of the individual patient's problems. The evaluation of a patient should always start with a thorough history and physical examination and proceed to a problem list and differential diagnosis. Only then will the physician be able to choose the proper laboratory studies in the proper sequence. While laboratory studies can be used to "screen" patients with vague symptoms and signs, they are best used to confirm a suspected diagnosis and/or to monitor the progress of a disease and its treatment. This paper summarizes some of the commonly used lab tests in these three areas by hormonal disorder. Diabetes Mellitus Insulin dependent diabetes mellitus (IDDM), the most common hormonal disorder seen in children, requires very little laboratory testing for diagnosis. A child presenting with a history of polyuria, polydypsia and weight loss, rarely requires more than a serum blood glucose and urinalysis (to confirm glucosuria and ketonuria) to make the diagnosis. At the initial presentation, measurement of serum ketones and electrolytes (Na, K, C1 and CO2) may be indicated to check for ketosis, acidosis or electrolyte imbalance. With the increasing number of adolescents Reprint requests : Michael J. Bourgeois, Department of Pediatrics, Texas Tech University School of Medicine, th Street, Lubbock, Texas pedmjb@ttuhsc.edu. presenting with what appears to be Type 2, or noninsulin dependent diabetes mellitus (NIDDM), and other forms of glucose intolerance, the diagnosis is not always clear. Still, the basic laboratory tests listed above suffice in most cases. Table 1 lists the criteria for diagnosing diabetes mellitus as recommended by the American Diabetes Association 1. While an oral glucose tolerance test (OGGT) is usually not necessary to diagnose Type 2 diabetes mellitus, it may be used to evaluate insulin secretion in relation to glucose levels. Rarely should a glucose tolerance test be done in a young person without measuring insulin. To maximize the usefulness of such testing, it is important to remember that patients need to be on a high carbohydrate diet for at least 3 days prior to the test. Other laboratory tests that could be done during the initial diagnosis of a patient with diabetes mellitus include anti-islet cell antibodies, anti-insulin auto- antibodies and C-peptide levels. While they have been valuable research tools, they offer little benefit in clinical practice. In established patients with diabetes mellitus, Types I and 2, routine self-blood glucose monitoring (SBGM) has become the standard of care of ongoing monitoring. Routine measurement of glycosylated hemoglobin (GHb), or HgAlc, every 3 to 4 months provides an independent assessment of ongoing blood glucose control. Along with the usual clinical monitoring, patients with diabetes should have periodic screening for proteinuria. Measurement of Indian Journal of Pediatrics, 2000; 67 (3) :

2 M.J. 8our0eoi$ TABLE 1. Criteria for the Diagnosis of Diabetes Mellitus ~ 1. Symptoms of diabetes plus casual plasma glucose concentrations > 200 mg/dl (11.1 mmol/1). Casual is defined as any time of day without regard to time since last meal. The classic symptoms of diabetes include polyuria, polydipsia and unexplained weight loss. or 2. Fasting plasma glucose >126 mg/dl (7.0 mmol/1). Fasting is defined as no caloric intake for at least 8 hours. or 3. 2-hour plasma glucose > 200 mg/dl (11.1 mmol/1) during an oral glucose tolerance test (OGTF). The test should be performed as described by WHO, using a glucose load containing the equivalent of 75-grams of anhydrous glucose dissolved in water. In the absence of unequivocal hyperglycemia with acute metabolic decompensation, these criteria should be confirmed by repeat testing on a different day. The third measure (OGT1O is not recommended for routine clinical use. Reproduced with kind permission of Editor, Diabetes Care 1999; 22 (Suppl-1). microalbumin on an annual or semi-annual basis is recommended 2. In older patients and those with family history of lipid abnormalities, periodic assessment of serum triglyceride and cholesterol levels is warranted. Disorders of the Thyroid Gland Laboratory evaluation of suspected or knownthyroid disorders is probably the most straightforward evaluation of endocrine disorders. Since the vast majority of cases of suspected or known hypothyroidism and hyperthyroidism are primary in origin, a TSH (thyroid stimulating hormone) and a total or free T4 (thyroxine) are usually adequate to confirm the diagnosis. Until recently, measurement of total T4 was the most readily available method of assessing thyroid hormone levels. Since most of the circulating T4 is protein bound, alterations in serum proteins can alter the results. Conditions such as pregnancy and the use of oral contraceptives can elevate total T4 levels, while low levels of proteins could give fictitiously low levels of thyroxine. While more difficult to perform and standardize, free T4 levels look to the unbound fraction of circulating thyroid hormone. It is not significantly affected by alterations in serum proteins 3. The RT3U (Resin T3 Uptake) has traditionally been used as an indirect assessment of thyroid binding globulin (TBG). In recent years, the availability of assays to directly measure TBG and free-t4 levels, have reduced the need for the RT3U. In patients who are clinically hyperthyroid but have normal or low T4 levels, measuring a total or free T3 level is indicated to confirm the diagnosis of T3 thyrotoxicosis. Table 2 summarizes the more 204 common pattern of thyroid function tests seen and their interpretation. When a patient is diagnosed with a thyroid disorder (hyothyroidism or hyperthyroidism), antibody testing is often done. This might include antiperoxidase antibodies and antithyroglobulin antibodies for suspected thyroiditis and thyroid-stimulating immunoglobulins (TSI) or long-acting thyroid antibodies (LATS) for suspected Grave's disease. While these may provide some insight into to the etiology of the patient's problem, they rarely change the course of treatment. While laboratory testing in primary hypothyroidism is fairly straightforward, the assessment of secondary (pituitary deficiency) or tertiary (hypothalamic dysfunction) is more subtle. Both should have a low T4 and TSH, but the results are usually equivocal. When necessary a thyrotropin releasing hormone (TRH) stimulation test can be performed. To do the test the patient is given a dose of TRFI ( 7 mcg/kg, maximum dose of 500 mug), intravenously, and blood is drawn at 30-minute intervals for 11,4 to 2 hours to determine the TSH, and sometimes, prolactin response. No rise in TSH'would suggest secondary hypothyroidism and a sustained rise would suggest tertiary hypothyroidism 4~. The same tests (T4 and TSH) are used to monitor the progress of treatment of hyperthyroidism, as wei1 as hypothyrodism. Obviously, optimal therapy aims to maintain both tests in the normal range. Because of the half-life of T4, it is rarely necessary to repeat thyroid function tests more than every 1 to 3 months. Even in infants with congenital hypothyroidism, where early treatment and normalization of T4 and TSH levels are crucial to maximize outcome, this interval is reasonable. The American Academy of Indian Journal of Pediatrics, 2000; 67 (3)

3 Optimal Use of Laboratory Tests in Pediatric Endocrine Practice TABLE 2. Patterns of Thyroid Function Tests & Interpretation TSH Total T4 Free T4 RT3 U TBG Total T3 Possible diagnoses E L L L N NorL N or L E E E N N or E NorL Nor L N or L N E N L L L N N L N E L N E N L E Primary hypothyroidism Primary hyperthyroidism T3 thyrotoxicosis Secondary/tertiary hypothyroidism; Sick thyroid syndrome TBG deficiency TBG excess; pregnancy; Oral contraceptives N = normal, E = elevated, L = low Pediatrics 6 recommends the following schedule for laboratory evaluations in children with congenital hypothyroidism : 1. At 2 and 4 weeks after beginning L-thyroxine treatment. 2. Every 1 to 2 months during the first year of life. 3. Every 2 to 3 months during the second to fourth years of life. 4. Every 3 to 12 months from the age of 3 years until the completion of growth. 5. At more frequent intervals when compliance is questioned or abnormal values are obtained. Vogiatzi and Kirkland 7 support these intervals during the first two years of life. They showed that infants started on mg of L-thyroxine, daily, required fewer dosage changes than infants started on mg daily. Disorders of the Adrenal Gland The normal circadian rhythm of ACTH and cortisol production and age of the patient, as well as the nature of the suspected disorder, must be kept in mind as one considers the laboratory evaluation of adrenal disorders. In children and adults, ACTH and cortisol levels are higher in the mornir~g than evening. Infants, especially newborns, who have not established a normal sleep-wake cycle, may not have the expected pattern. In hypoadrenalcorticism, one expects to find low levels of cortisol all through the day. If the condition is due to primary failure of the gland (i.e., Addison's disease), ACTH levels will be markedly elevated. There may also be co-existing electrolyte imbalance (hyponatremia and hyperkalemia). If the condition is secondary (pituitary insufficiency) or tertiary (hypothalamic dysfunction), ACTH levels will be low. In either case an ACTH.simulation test will show a lack of rise in cortisol. This is done by giving 0.25 to 1 mg of synthetic ACTH 1-24 (cortrosyn), intravenously, over 1 to 2 minutes, and measuring cortisol levels every 30 minutes for 2 hours s. This test has also been recommended as a way to evaluate adrenal recovery from long-term suppression due to high dose glucocorticoid therapy. In evaluating a child for endogenous glucocorticoid excess, a comparison of a morning and evening cortisol level has been the traditional screening method. Normally, the morning level is expected to be 2 or 3 times higher than the evening level. A more definitive test is to give the patient dexamethasone - low dose (1 mg) one day and high dose (8 rag) the next. Classic Cushing's disease (due to excess pituitary secretion of ACTH) is indicated when the cortisol level is suppressed by the high dose, but not the low dose of dexamethasone. Lack of suppression, even with high dose dexamethasone is highly suggestive of an adrenal tumor 9. Any form of endogenous Cushing's syndrome will usually require some form of surgical intervention which, if successful, may result in the need for subsequent glucocorticoid replacement. Improvements and refinements of immunoassay techniques have made the assessment of problems of adrenal androgen excess (congenital adrenal hyperplasia, virilizing tumours, etc.) much easier and more straightforward than past years. No longer is it necessary to collect 24-hour urine specimen of an infant. For most patients, measurements of serum or plasma 17-alpha-hydroxyprogesterone, androstenedione, testosterone, and sometimes, dehydroepiandrostenedione (DHEA) are sufficient in evaluating children for Indian Journal of Pediatrics, 2000; 67 (3) 205

4 M.J. Bourgeois excessive premature virilization. Occasionally, measuring these hormones before and after a dose of ACTH is more helpful in defining the problem 8. A child with ambiguous genitalia presents the pediatrician with a social and possibly medical emergency. Immediate evaluation is needed to determine the appropriate gender assignment as well as the need for glucocorticoid replacement. While it is not a laboratory test, per se, a pelvic ultrasound is an excellent first step. The demonstration of a uterus, especially if ovaries are identified, clearly indicates the appropriate gender assignment and virtually assures that the child will have congenital adrenal hyperplasia. Initial laboratory tests would include a chromosomal analysis (to confirm chromosomal sex) and serum or plasma 17-alpha-hydroxyprogesterone, androstenedione and testosterone. In confirmed cases of CAH on treatment, these same tests are used to monitor treatment. The goal is to keep them suppressed while allowing the child to grow normally and avoiding Cushingoid features. Disorders of Puberty Children presenting with abnormalities of puberty, both early and late, are among the most challenging problems to assess. This is because random laboratory testing is often equivocal and difficult to interpret. As children enter puberty, gonadotropin levels (LH and FSH) fluctuate dramatically and are not good discriminators of pubertal stage. Gonadal hormones (estrogen and progesterone) in girls also cycle and "normal reference values" are different for different stages of the menstrual cycle. In boys, testosterone levels tend to be more stable, but still show wide ranges of normalcy in early puberty. Before deciding on any particular laboratory tests it is helpful to consider what hormones are responsible for what secondary sex characteristics and the sequence in which they are expected to occur. In girls, estrogen is the main pubertal hormone and is responsible for breast development, fat redistribution, increased vaginal secretions and uterine changes, which ultimately led to menarche. Pubic and axillary hair, seen in girls at puberty time, are androgen mediated, much of which is produced by the adrenal gland. While some "normally developing" girls will have onset of pubic hair before breast development, girls with precocious puberty almost always have estrogen effect (breast development, vaginal secretions or bleeding, etc.) 206 before pubic hair appears. A girl presenting with early pubic hair development, especially if other signs of virilization (clitoromegaly, etc.) are seen, should be evaluated for androgen excess before being worked up for precocious puberty. When true central precocious puberty is suspected, serum or plasma levels of LH, FSH, and gonadal hormones (estradiol in girls and testosterone in boys) are often measured. Unfortunately, the results are usually equivocal. Definitive testing is usually in the form of a gonadotropin releasing hormone (GnRH) stimulation test. The patient is given a dose of a gonadotropin releasing hormone analog and serial levels of LH and FSH are done. The higher the response, the further into puberty is the patient. In confirmed cases of central precocious puberty, treatment with gonadotropin releasing hormone analogs (GnRHa) has been shown to be effective. In treated girls the GnRH stimulation test should be repeated periodically to confirm that puberty is being suppressed. In boys, periodic testosterone levels appear adequate to monitor treatment. Disorders of Growth The laboratory can be a valuable adjunct to the evaluation of a child with poor, or attenuated growth, provided the physician performs a thorough history and physical examination and carefully contemplates the differential diagnosis. A better approach would be to consider what, if any, lab tests would be beneficial based on the patient's history and physical findings. The first step is to ensure that the child does indeed have a growth problem. No laboratory testing is needed for this step. Just a good history, including growth records, and b)hysical exam. What is the child's growth velocity? Does the child have growth failure (growth velocity less than 2SD below the mean) or short stature with normal growth velocity? What is the family history? Are there any clinical clues that point to a specific endocrine or nonendocrine disorder (dysmorphic features, specific signs and symptoms, septo-optic dysplasia, etc.)? Obviously specific clues should be pursued with appropriate testing. The second step would be to decide if tests are necessary to look for organ system problems. Some organ system disorders are fairly obvious (cyanotic heart disease, severe pulmonary problems, severe Indian Journal of Pediatrics, 2000; 67 (3)

5 Optimal Use of Laboratory Tests in Pediatric Endocrine Practice TABLE 3 : Commonly Used Provocative Agents for Growth Hormone Testing Provocative Dose and Blood samples route of administration (in minutes) Arginine 0.5 gm/kg, maximum of 40 gm 0, 30, 60, 90 given IV over 30 minutes Insulin to 0.1 U/kg 0, 15, 30, 60 given IV bolus Clonidine 5 mcg/kg, maximum of 250 mcg 0, 60, 75, 90, 120 L-Dopa 500 mg for weights over 30 kg 0, 40, 60, 90, mg for weights 15 to 30 kg Glucagon 1 rag, given IM or SQ 0, 60, 120, 180, 240 neurological dysfunction, etc.) Others may be less obvious. Is there reason to suspect renal, hepatic, or hematological dysfunction? Could the child's poor growth be an early sign of evolving inflammatory bowel disease? What is the child's nutritional status and could it account for the child's poor growth? Obviously, the answers to these questions would determine if there was a need to do a complete blood count, serum electrolytes, liver function studies, or sedimentation rate. The third step is the endocrine screening and/or testing. In children with growth failure in the absence of significant organ system disease, the first endocrine test probably should be thyroid function tests (T4 and TSH). While most patients with hypothyroidism may have some signs and symptoms, others may not. In short, poorly growing girls, an additional consideration is a chromosomal analysis to check for the possibility of Turner syndrome. Once all of the above considerations have been exhausted the question about growth hormone is left. Because of the circadian rhythm of growth hormone secretion, random measurements of growth hormone levels are rarely helpful. As a screening test, some practitioners have used rigorous exercise in children who are old enough to cooperate m. The child must be fasting for at least four hours and run, climb stairs, or ride an exercise bike 11 for at least 20 minutes. Growth hormone levels are drawn before and immediately after the exercise. Depending on the assay, a growth hormone level greater than 7 or 10 ng/dl is considered normal. Insulin-like growth factor-1 (IGF-1), or somatomedin-c is a commonly used screening test. However, it is important to recognize that normal Indian doumal of Pediatrics, 2000; 67 (3) values vary with age and pubertal status. Even mild states of undernutrition will lower the level significantly. Recently, the measurement of insulinlike growth factor binding protein-3 (IGFBP-3) has been recommended as a diagnostic too11% In spite of increasing controversy, there is some fairly universal agreement that the most definitive way to diagnose growth hormone deficiency is to demonstrate poor growth hormone secretion in response to two provocative agents 13. Table 3 lists some of the more common provocative agents, recommended doses, and the commonly recommended blood sampling intervals. There are differing opinions as to whether or not a priming agent-estrogen, testosterone, or propranolol-should be used prior to the stimulation test la. The cutoff value for growth hormone levels that define "normal" from growth hormone deficiency has changed over the years and varies from author to author. It is important to know the type of assay and the normative values of the laboratory you will be using. Children on growth hormone treatment for growth hormone deficiency should be monitored carefully. Routine laboratory testing is not usually needed, but a child with a waning response to exogenous growth hormone should have thyroid function tests done. An occasional IGF-1 level may help assess compliance and adequacy of the dose being used. The child with accelerated growth presents a similarly diverse set of considerations. What is the child's age and family history? Is the child obese and tall on that basis? Are there sirens of early sexual development indicating precocious puberty, excessive androgen production, etc.? When growth hormone excess is suspected, a 207

6 M.J. Bourgeois serum IGF-I level is a good first step. It is usually markedly elevated in pituitary gigantism. Confirmation is obtained by measuring growth hormone levels during an intravenous or oral glucose tolerance test. A normal response shows the growth hormone levels to be suppressed, while growth hormone excess shows elevated levels 13. Diabetes Insipidus As discussed earlier, the initial laboratory test of a child with polyuria and polydypsia should be a urinalysis and, possibly, a blood glucose to check for diabetes mellitus. If there is no glucosuria, obviously the polyuria cannot be explained on the basis of elevated blood glucose levels. The differential diagnosis shifts to some form of psychogenic or compulsive water drinking, renal disease, or diabetes insipidus (central or nephrogenic). Obviously, a thorough history is important. Does the increased drinking and urination occur only during the day, or day and night? When did it start? Is there a family history? Is there some pre-existing condition which predisposes to one of these diagnosis? Is there a history of hypernatremic dehydration? There is no simple screening test for diabetes insipidus. A high random urine specific gravity or osmolality is good evidence against diabetes insipidus, but is not always present. Obtaining an early morning urine specimen may improve the yield, but must be done with some caution. Children with true diabetes mellitus are rarely able to tolerate fluid restriction overnight. Serum electrolytes are rarely helpful, except in the presence of obvious dehydration. The diagnosis of diabetes insipidus requires the demonstration of a low, or relatively low, urine osmolality in the face of an elevated or rising serum osrnolality 14. The distinction between central and nephrogenic DI is the responsiveness to exogenous pitressin or DDAVP. Traditionally, this has been accomplished by doing a water deprivation test. Under controlled conditions, the patient is deprived of water and sequential measurements are made of vital signs, weight, urine output and osmolality, and serum osmolality and electrolytes. The normal response would be a decrease in urine output with an increase in urine osmolality. If this does not happen the patient is given a dose of aqueous pitressin or DDAVP, at or before the onset of signs of dehydration (weight loss, rise in serum osmolality and sodium, etc.). A clear response (decrease in urine output with a rise in urine osmolality) indicates the presence of central diabetes insipidus (ADH deficiency). No response would support the diagnosis of nephrogenic diabetes insipidus. In recent years, the availability of direct assays for antidiurectic hormone have helped in the work up of these patients. In stable patients on treatment, usually periodic measurements of urine output and specific gravity at home are the only lab needed monitoring. During illnesses and periods of poor control, closer attention will need to be given to the clinical and laboratory findings of dehydration and electrolyte imbalance. Hypoglycemia The differential diagnoses and course of evaluation, of a child with hypoglycemia varies with age and circumstances of the episode. In the newborn period potential causes include transient neonatal hypoglycemia, hyperinsulinism (as in neisidioblastosis, islet cell hyperplasia etc.), hormone deficiencies (growth hormone, adrenal etc.), and metabolic disorders (glycogen storage disease, galactosemia etc.). In older children, the incidences of hormonal deficiencies and ketotic hypoglycemia increase while that of metabolic disorders decrease. As with other conditions, the laboratory evaluation of a patient with hypoglycemia should be decided upon after considering the clinical picture. For infants, one needs to see if the mother has diabetes, is there macrosoma, hepatomegaly, etc.? Did the hypoglycemia start immediately or develop after feedings started? For an older child, are there signs suggesting a hormone deficiency, such as poor growth, weakness, gastrointestinal upset, hyperpigmentation, etc.? An episode of spontaneous hypoglycemia offers its own opportunity for evaluation. Blood samples taken during such an episode can be assayed for glucose," insulin, growth hormone, and cortisol. Elevated insulin levels in the face of low glucose levels is suggestive of hyperinsulinemia. Low growth hormone and/or control levels in the face of hypoglycemia and low insulin levels would suggest these hormonal deficiencies. Additional lab studies, 208 Indian Journal of Pediatrics, 2000; 67 (3)

7 Optimal Use of Laboratory Tests in Pediatric Endocrine Practice such as urine for ketones, serum lipids and free fatty acids should be considered. In older patients a controlled fast can be undertaken. As the fast proceeds, periodic blood samples are taken to measure glucose, insulin, and free fatty acids. As hypoglycemia occurs additional samples for growth hormone and cortisol can be done. REFERENCES 1. The Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. Report of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. Diabetes Care 1999; 22 (Supp. 1) : $5- $ American Diabetes Association. Standards of medical care for patients with diabetes mellitus. Diabetes Care 1999; 22 (Supp. 1) : $32-$ Lee P and Reiter E. Endocrine testing principles and procedures. In: Kappy MS, Blizzard RM and Migeon CJ (eds). Wilkins: The Diagnosis and Treatment o t Endocrine Disorders in Childhood and Adolescence, Springfield: Charles C. Thomas, 1994; Foley T, Matvaux P and Blizzard R. Thyroid disease. In: Kappy MS, Blizzard RM and Migeon CJ (eds). Wilkins: The Diagnosis and Treatment of Endocrine Disorders in Childhood and Adolescence, Springfield : Charles C. Thomas, 1994 : Moore WT and Eastman RC. Thyroid diseases. In : Moore WT and Eastman RC (eds). Diagnostic Endocrinology, Philadelphia : B.C. Decker Inc., 1990 : American Academy of Pediatrics. Newborn screening for congenital hypothyroidism: recommended guidelines. Pediatrics 1993; 91 : Vogiatzi MG and Kirkland JL. Frequency and necessity of thyroid functions tests in neonates and infants with congenital hypothyroidism. Pediatrics 1997; 100 (3) : E6 (electronic article). Conclusion The laboratory is a useful adjunct to the evaluation of infants, children and adolescents with suspected endocrinological problems. With careful consideration of the patient's history and physical findings, the appropriate course of evaluation can be decided upon. 8. Migeon CJ and Donohoue PA. Adrenal disorders In : Kappy MS, Blizzard RM and Migeon CJ (eds) Wilkins: The Diagnosis and Treatment of Endocrine Disorders in Childhood and Adolescence, Springfield: Charles C. Thomas, 1994 : Kamilaris TC and Chrousos GP. Adrenal diseases. In : Moore WT and Eastman RC (eds). Diagnostic Endocrinology, Philadelphia : B.C. Decker Inc : Johanson AJ and Morris GO. A single growth hormone determination to rule out growth hormone deficiency. Pediatrics 1977; 59 : Seip RL, Weltman A, Goodman D et al. Clinical utility of cycle exercise for the physiological assessment of growth hormone release in children. Am l Dzs Child 1990; 144 : Blum WF, Ranke MB, Keitzmann K et al. A specific radioimmunoassay of the growth hormone dependent somatomedin binding protein: its use for diagnosis of growth hormone deficiency. J Clin Endocrin Metab 1990; 70 : Blizzard RM and Johanson A. Disorders of growth. In: Kappy MS, Blizzard RM, and Migeon CJ (eds.) Wilkins: The Diagnosis and Treatment of Endocrine Disorders in Childhood and Adolescence, Springfield : Charles C. Thomas, 1994 : Bichet DG. Diabetes insipidus and vasopressin. In : Moore WT and Eastman RC (eds.) Diagnostic Endocrinology, Philadelphia : B.C. Decker Inc., 1990 : Indian Journal of Pediatrics, 2000; 67 (3) 209