Our knowledge of hypothalamicpituitary function has increased rapidly during the last decade due to the radioimmunoassay technique for pituitary hormones and the isolation of the hypothalamic releasing hormones. Immunoassays for the estimation of thyroid-stimulating hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), and human growth hormone (HGH) in plasma are easily available for clinical use.
The existence of substances in the hypothalamus that regulate the function of the pituitary gland has been postulated for many years.1 The first of these hypophysiotropic factors or hormones to be isolated2 and synthesized was thyrotropic releasing hormone (TRH). Synthesis was accomplished by many research groups, and the hormone has been available for clinical use since 1971. In 1971 a synthetic luteinizing hormone-releasing hormone (LH-RH) was introduced in clinical work by Schally et al.3 Since then a number of studies on this hormone have been performed.
Research indicates the existence of further releasing hormones: growth hormone releasing hormone (GHR), corticotropic releasing hormone (CRH), follicle-stimulating hormone releasing hormone (FSH-RH), and prolactin releasing hormone (PRH). These hormones, however, are not yet available for clinical use.
From animal experiments it has been shown that the releasing hormones are produced in different parts of the hypothalamus and are transported to the pituitary gland by the stalk portal vessels. In the pituitary the substances release stored hormones and probably stimulate the production of more hormone.
From a theoretical point of view, the exogenous administration of releasing hormones should tell us whether a lesion is located in the hypothalamic area or in the pituitary. An increase in pituitary hormone after administration of releasing hormone would indicate a hypothalamic lesion; absence of response would indicate that the lesion is located in the pituitary gland, which is unable to release hormone. However, clinical studies have shown that there is further information to be received from these tests.
Because thyrotropic releasing hormone (TRH) and luteinizing hormonereleasing hormone (LH-RH) are available for clinical purposes, the applicability of the TRH and LH-RH tests in pediatric practice will be discussed in this article. Clinical studies have shown that TRH stimulates the release of TSH and prolactin and that LH-RH stimulates the release of LH and FSH.
A number of slightly different procedures have been proposed for the TRH and LH-RH tests; however, most authors suggest the following routine schedule.
TRH test. The test is performed in the morning after an overnight fast. An indwelling vein needle is inserted and is kept open by a slow infusion of isotonic saline. TRH is administered in a bolus injection at time 0, and blood samples for the analysis of TSH are drawn through the needle at -10, 0, +10, +20, +30, +60, and +120 minutes. The recommended amount of TRH is 200 meg.
Figure 1. TSH response to synthetic TRH in normal subjects 0-0 (mean and S. E. M) and in two children with hyperthyroidism Δ - Δ.
Figure 2. TSH response to synthetic TRH in hypothalamic-pituitary disorders. 0-0 isolated growth hormone deficiency.
LH-RH test. As in the TRH test, the LH-RH test is performed in the morning after an overnight fast. Blood samples for the assay of LH and FSH are drawn at -10, 0, and +10, (+15), +20, +30, +60, and +120 minutes after a bolus injection of LH-RH. The standard amount of LH-RH injected is 100 meg. of synthetic LH-RH. However, a variety of dosages have been tried (5 to 10 meg./m.2, 25 meg./m.2, and 50 meg. /m. 2 of body area).
Figure 3. Maximal LH response to LH-RM in boys of different pubertal stages according to Tanner.
Figure 4. Maximal FSH response mean + S.D. to LH-RH in boys of different pubertal development according to Tanner.
In addition to separate TRH and LH-RH tests, it seems possible to combine both tests by administering TRH and LH-RH in succession.
The analyses of TSH, LH, and FSH have to be performed with great accuracy. As the standards of the different hormones vary among laboratories, it is necessary to be aware of the actual standards used. Furthermore, there are differences depending on the radioimmunoassay technique used. Thus, it is difficult to compare the absolute figures received from different laboratories. Most laboratories, however, by now have great experience in control subjects. In the following figures TSH, LH, and FSH are determined by a radioimmunosorbent technique.4
The standards were: TSH- hTSH Research standard A(NIMR), London; LH - 1 ng. was equivalent to 83 ng. of LER 907; FSH- 1 ng. FSH was equivalent to 369 ng. of LER 907.
PRACTICAL APPLICATION OF THE TRH TEST
Effect of TRH in normal subjects. Figure 1 shows the effect of TRH in normal subjects (0-0). Within 30 minutes an increase in plasma TSH appears, which declines within 60 minutes. It has been shown that the response is greater for women than for men. The increased response in women seems to be more pronounced in the preovulatory phase than in the postovulatory phase. No sex difference seems to exist in children before puberty.
Effect of TRH in patients with hyperthyroidism. The clinical diagnosis of hyperthyroidism, especially in pubertal girls, is often venturous, and the interpretation fo T:3 or T:4 tests is difficult. The basic TSH levels in primary hyperthyroidism are low, and there is no increase in TSH during the TRH test. (Figure 1 Δ - Δ ).
TRH tests in patients with hypothalamic-pituitary disorders. The TSH response to TRH in patients with hypothalamic-pituitary disorders shows a great variety. In Figure 2 the results in some patients studied are shown. Most patients with isolated growth hormone deficiency show a normal response. Patients with TSH deficiency show an increase in TSH in surprisingly high frequency, indicating that the defect in most patients is of hypothalamic origin. However, a high number of these patients show a delayed increase (the 60 minute value is higher than the 20 minute value). This might be due to a chronically unstimulated pituitary that has to be treated with repeated injections of TRH to obtain a normal response. Only a few children with pituitary tumors have been studied; in most patients the response to TRH was normal.
No major toxic effects of TRH have been reported; however, a rather common phenomenon experienced by our patients was transient nausea and an urge to micturate.
To summarize, the TRH test is useful in discriminating between hypothalamic and pituitary forms of TSH insufficiency. The hypothalamic lesion often is characterized by a delayed TSH increase. Furthermore, the TRH test seems to be a valuable contribution in the diagnosis of hyperthyroidism.
As stated earlier, an increase in prolactin has been demonstrated on I.V. injection of TRH. The experience in children, however, is insignificant.
Effect of LH-RH in normal subjects. The response to LH-RH seems to be correlated to pubertal development. Roth et al.5 recently have suggested that "an ordered sequential maturation of the hypothalamic-pituitary gonadotropin-gonadal axis presages the onset of puberty in man." Figure 3 shows the increase in LH in normal boys in different stages of puberty, and Figure 4 shows the increase of FSH in the same patients. Corresponding studies in normal girls are few. However, the LH response seems to be higher in boys and the FSH response seems to be higher in girls.
Effect of LH-RH in delayed and precocious puberty. In patients with delayed or precocious puberty the response to LH-RH corresponds to pubertal development. Thus, boys with delayed puberty show a low LH-FSH increase according to age, and patients with precocious puberty show an increase similar to adults.
Effect of LH-RH in patients with testicular deficiency. Several studies have shown elevated basal levels of LH in patients with Klinefelter's syndrome or anorchia. Their response to LH-RH is significantly higher than in controls (Figure 5 0-0).
Figure S. Exaggerated LH response to LH-RH in a 12-year-old boy with Klinefelter's syndrome 0-0 and in an eight-year-old girl with Turner's syndrome Δ - Δ- Delayed LH response in a 15-year-old boy with olfacto-genitai syndrome a - D.
The shadowed area indicates normal response according to pubertal development of the tested children (Tanner stage 1).
Effect of LH-RH in patients with ovarian deficiency. Studies on patients with Turner's syndrome have shown increased levels of LH in early puberty. Preliminary investigations have shown that the response to LH-RH is already increased at the age of eight years (Figure 5 ? - ?).
Effect of LH-RH in hypothalamic-pituitary disorders. The experience of the LH-RH test as a discriminator between hypothalamic and pituitary disorders varies among different studies. Mortimer6 claims that it is impossible to distinguish between hypothalamic and pituitary causes of hypogonadism. Other authors7'8 have found the LHRH test suitable for this purpose. However, as in the TRH test, a chronic lack of pituitary stimulation by releasing hormones seems to decrease the response to releasing hormones. In patients with isolated growth hormone deficiency the increase in LH and FSH is similar to the increase seen in prepubertal controls.
Figure 6. Urinary LH response to LH-RH in boys of different pubertal stages according to Tanner.
Effect of LH-RH in patients with olfacto-genital syndrome (Kallmann syndrome). In this disease, characterized by secondary hypogonadism with absence of androgens and with congenital anosmia, an isolated lack of gonadotropin is present. LH-RH test has been performed in two boys aged 14 and 15 years. They showed no pubertal development whatsoever. Basal levels of LH and FSH were very low; however, a significant but delayed increase in LH was caused by the LH-RH test (Figure 5). The delayed response may be explained by lack of earlier stimulation of the pituitary. This would indicate that the defect in the olfacto-genital syndrome is located in the hypothalamic area.
MODIFICATION OF THE LH-RH TEST
LH estimation in urine. As it is possible to estimate LH excretion in urine and as the renal clearance for LH seems to be rather rapid, it is possible to study the LH response to LH-RH in urine.' Urine is collected four hours prior to and four hours following the I.V. injection of 50 to 100 meg. of LH-RH. Figure 6 shows the increase in LH excretion in urine in boys of different pubertal development. This modification seems to be suitable in patients in whom there are difficulties in proper blood sampling.
Intranasal administration of LH-RH. Recently, London and collaborators10 haye published preliminary data on hormonal response to intranasal LHRH. The intranasal administration is similar to the administration of Iysinevasopressine given for treatment of diabetes insipidus. A proper intranasal administration of I.V. LH-RH was able to cause an increase of LH and FSH in good agreement to the traditional I.V. route.
To summarize, the response to LHRH appears to be useful in distinguishing between delayed puberty, primary testicular or ovarian failure, and primary hypothalamic-pituitary failure, not only in puberty but also before puberty.
In women with hypothalamic hypogonadism, repeated injections of LHRH have increased LH levels in sufficient amount to induce ovulation. This is not of interest in pediatric practice; however, we might speculate on the possibility of synthesizing growth hormone releasing hormone in sufficient amounts to cause a constant and therapeutic increase of growth hormone. This would give us the possibility of treating all those patients with growth hormone deficiency who now are untreated due to insufficient sources of growth hormone.
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9. Westphal, O. and Wide, L. Acta Ped. Scand. To be published.
10. London, D. R.. Butt, W. R.. Lynch, S.S.. Marshall, J. C, Owusu, S.. Robinson, W. R., and Stephanson, J.M. Intranasal LH-releasing hormone. J. Clin. Endocrinol. Metabol. 37 (1973), 829.