Growth hormone, prolactin and thyrotrophin responses to thyrotrophin-releasing hormone in diabetic patients

The endocrine system in diabetes mellitus

The pathophysiology of diabetes mellitus is complex and not fully understood. However, it emerges as an abnormal metabolic condition associated with a systemic damage to the vascular bed. Cumulative evidence also reveals that the endocrine system is not intact in patients with diabetes mellitus. It is not clear whether the changes observed in the endocrine system represent a primary defect or reflect the effects of the impaired insulin action and abnormal carbohydrate and lipid metabolism on the hormonal milieu. Review of the literature reveals that the function of the entire endocrine system including the functions of hormones from the hypothalamus, pituitary, adrenal, thyroid, parathyroid, the vitamin D system, the gonads, and the endocrine function of the adipose tissue, is impaired. Good metabolic control and insulin treatment may reverse some of these abnormalities. It remains unanswered as to what extent these changes in the endocrine system contribute to the vascular pathologies observed in individuals affected by diabetes mellitus and whether part of the abnormalities observed in the endocrine system reflect a basic cellular defect in the diabetic syndrome.

Effects of improved blood glucose on insulin-induced hypoglycaemia, TRH, GnRH and exercise tests in insulin-dependent diabetes

In order to evaluate effects of metabolic control on pituitary function in insulin-dependent diabetes exercise, hypoglycaemia (insulin tolerance test), thyrotrophin releasing hormone and gonadotrophin releasing hormone, tests were performed on 25 patients before (Study 1) and after 2 weeks of improved metabolic control (Study 2). Patients were sub-divided into C-peptide negative (CpN, 10 patients with no residual C-peptide secretion) and C-peptide positive (CpP, 15 patients with residual beta-cell function) groups for analysis of results. Exercise induced higher growth hormone responses in CpN patients independent of metabolic control (P less than 0.001). Thyrotrophin releasing hormone induced higher growth hormone responses in CpN patients; this response was threefold greater after improved control (P less than 0.005). Growth hormone and cortisol response to hypoglycaemia and thyroid stimulating hormone and prolactin secretion in response to thyrotrophin releasing hormone were unaffected by residual beta-cell function or metabolic control. Luteinizing hormone response to gonadotrophin releasing hormone in CpN patients was impaired and lower after improved control (P less than 0.002). The results indicate an association between residual pancreatic insulin secretory and hypothalamic/pituitary function, possibly reflecting central neurosecretion of insulin.

The effect of residual beta cell activity on menstruation and the reproductive hormone profile of insulin-dependent diabetics

To investigate the cause of secondary amenorrhoea in insulin-dependent diabetes gonadotrophins, sex steroid hormone levels and residual beta cell activity (C-peptide index) were estimated in a group of 43 women with IDDM. Among 26 women with residual insulin secretion, the C-peptide positive (CpP) group, 5 had secondary amenorrhoea (CpP-Am); among 17 women without endogenous beta cell activity, the C-peptide negative (CpN) group 6 had secondary amenorrhoea (CpN-Am). In this study two different types of secondary amenorrhoea in insulin-dependent diabetics were observed. All CpP-Am women have the classical hormone profile of the polycystic ovary syndrome (increased (LH/FSH ratio, increased serum testosterone, decreased SHBG) together with a history of oligomenorrhoea and excess weight before the onset of diabetes. On the other hand, all CpN-Am women had decreased LH levels as well as low LH/FSH ratio and testosterone levels. These results strongly suggest that a lack of residual pancreatic beta cell activity influences hypothalamus-pituitary function in insulin-dependent diabetes. It might be concluded that PCOS is independent of diabetes while low LH amenorrhoea seems to be the consequence of diabetes and is strongly associated with a lack of residual insulin secretion.

Decreased thyroidal response to thyrotropin in type II diabetes mellitus

The responses to TRH and bovine TSH (bTSH) were compared in 19 men with uncontrolled type II diabetes mellitus and eight healthy control subjects. Baseline serum TSH, T3 and T4 were similar in both groups and the rise of serum TSH, T3 and T4 following the intravenous (IV) administration of TRH (500 micrograms) was not significantly different. Diabetic subjects showed a blunted response to the subcutaneous (sc) administration of bTSH (5 U) when their maximal serum T3 and T4 values were compared with controls (T4, 9.4 +/- 0.3 v 12.3 +/- 1.1 micrograms/dL, P less than .005; T3, 185 +/- 9 v 233 +/- 17 ng/dL, P less than .025; diabetic v control). When the response to bTSH was examined in seven patients after 4 to 5 days of strict glycemic control, the maximal T3 response was found to increase in six, and the maximal T4 response in five. These data show that the thyroidal secretory response to large doses of TSH is decreased in uncontrolled diabetes mellitus and that strict glycemic control frequently improves the response.

Arginine induced growth hormone (hGH) response and paradoxical hGH secretion stimulated by TRH in diabetes mellitus

Growth hormone (hGH) reserve following arginine administration and the paradoxical hGH response to thyrotropin-releasing hormone (TRH) were studied in 30 diabetics without evidence of vascular complications. The diabetics were divided into 4 groups according to the type of their disease and to the metabolic condition within the IDDM group (insulin-dependent: IDDM, in acceptable response and in poor metabolic control; non-insulin-dependent: NIDDM, and juvenile diabetics not requiring insulin at least for two years after diagnosing their disease: NIDDY). The results were compared with controls of identical age and normal weight. A paradoxical hGH response to TRH stimulation was found only in IDDM patients in poor metabolic control. In this group the hGH reserve revealed by arginine was significantly larger than in the others. It was shown that the induced hGH release was independent of the sex distribution of the groups and of the basal hGH values. Magnitude of the hGH reserve and appearance of the paradoxical hGH response were not necessarily correlated but the substantial reserve was frequently associated with a paradoxical response. It can be assumed that the unfavorable metabolic condition is of decisive importance in giving rise to these anomalies. Our observations seem to confirm the need for good metabolic control if the pathological hGH secretion in diabetics is to be prevented.

Effects of intravenous TRH on growth hormone and cortisol serum levels in children and adolescents with insulin dependent diabetes mellitus

The effects of an iv thyrotropin releasing hormone (TRH) bolus on serum growth hormone (GH) and cortisol levels were evaluated in 59 children and adolescents with insulin dependent diabetes mellitus (IDDM) and in 24 healthy, age-matched control subjects. In the IDDM group GH baseline levels sharply rose within 30 min after TRH and successively normalized. On the contrary, TRH injection failed to affect GH serum concentrations in the control group. The GH increase after TRH in IDDM patients was positively correlated to age, but unrelated to other variables, such as sex, pubertal stage, duration of disease, glycemia, glycosylated hemoglobin, thyrotropin and T4 concentrations. Twenty-one out of 59 diabetics and only 1/24 controls exhibited a paradoxical GH response to TRH, arbitrarily defined as a precocious increase (within 30 min), of more than 100% with respect to the baseline value, associated with a GH peak greater than 10 ng/ml. Eighteen IDDM patients underwent a second TRH test 12 to 24 months later and substantially exhibited the same GH pattern documented the first time. The mechanism responsible for such anomalous GH responsiveness to TRH in IDDM is unclear. However, it cannot be attributed to a nonspecific stress reaction, as proven by the lack of a concomitant increase of cortisol serum levels in the same subjects.

Impaired thyrotrophin secretion following the administration of thyrotrophin-releasing hormone in type II diabetes mellitus

Serum thyrotrophin has been measured before and after the intravenous administration of 200 micrograms of thyrotrophin-releasing hormone in 91 white subjects (33 stable diabetic patients and 58 healthy controls), none of whom had any clinical evidence of thyroid or pituitary dysfunction. Seven of the diabetic subjects failed to achieve a rise of serum thyrotrophin of greater than 2 mU/l above basal concentrations, as compared with only one of the control subjects (P = 0.006). The difference in response between diabetics and controls was confined to patients with Type II (non-insulin-dependent) diabetes: thus 5 of 13 Type II patients and 2 of 20 Type I (insulin-dependent) patients failed to show a normal response to thyrotrophin releasing hormone injection. No significant effect of glycaemic control on thyrotrophin responses was noted. These results suggest that Type II diabetes mellitus may be a cause of impaired thyrotrophin secretion in patients with no clinical evidence of pituitary disease. The mechanism for this impaired pituitary hormone release remains to be clarified.

The GABAergic control of prolactin release is not affected by insulin-dependent diabetes mellitus: evidence from studies with sodium valproate

In order to evaluate the influence of the GABAergic system in the regulation of PRL secretion in patients with IDDM, serum PRL levels were measured in 7 diabetics and in 7 normal men with sodium valproate (400 mg per os), a drug capable of increasing cerebral GABA concentrations. A significant decrease of serum PRL concentrations was observed between 30 and 120 min after sodium valproate administration in both control and diabetic subjects. The time course and magnitude of the sodium valproate effect were similar in all subjects. These data confirm the inhibitory control of the GABAergic system on PRL secretion in man as evidenced by the GABAergic drug sodium valproate. It is suggested that this system is not altered in diabetic patients.