Hypothalamic, pituitary and sex hormones

Chapter 38 Hypothalamic, pituitary and sex hormones




Synopsis




Figure 38.1 shows the hypothalamo-pituitary axes. The hypothalamus and pituitary glands form the centre of the ‘endocrine orchestra’. We will here describe the hypothalamic releasing hormones, and the anterior and posterior pituitary gland hormones, and drugs that are used to manipulate these axes.



These hormones, analogues (agonists) and antagonists can be used:



The scope of the specialist endocrinologist continues to increase in amount and in complexity and only an outline is appropriate here.



Hypothalamic and anterior pituitary hormones


The hypothalamus releases a number of locally active hormones that stimulate or inhibit pituitary hormone release (see Fig. 38.1).


The t½ of the polypeptide and glycoprotein hormones listed below is 5–30 min; they are digested if swallowed.













Growth hormone, somatrophin

(Genotropin, Humatrope) is a biosynthetic form (191 amino acids) of growth hormone prepared by recombinant DNA technology, as is somatrem. Naturally occurring human growth hormone was extracted from cadaver pituitaries and its supply was therefore limited. In 1985 the use of natural growth hormone was terminated because of the risk of transmitting Creutzfeldt–Jacob disease, the fatal prion infection. Growth hormone acts on many organs to produce a peptide insulin-like growth factor IGF-1 (somatomedin), which causes muscle, bone and other tissues to increase growth, i.e. protein synthesis, and the size and number of cells.


Growth hormone is approved for treatment of children with short stature due not just to growth hormone deficiency, but also to Turner’s syndrome, renal failure, small size for gestational age, Prader–Willi syndrome1 and, most recently, idiopathic short stature. Treatment is continued until closure of the epiphyses. Subsequent treatment into adulthood is also warranted where UK National Institute for Health and Clinical Excellence (NICE) guidelines are fullfilled. Growth hormone therapy should be confined to specialist clinics.


The use of growth hormone in adults varies among different countries. In the UK, treatment is limited to growth hormone-deficient patients with severely impaired quality of life. Treatment improves exercise performance, increases lean body mass and overall quality of life. A low starting dose of 0.27 mg s.c. daily is used, and adjusted at 4–6 week intervals according to clinical response and IGF-1 levels. It is recommended by NICE that treatment be discontinued in patients when quality of life improves by fewer than seven points on the Adult Growth Hormone Deficiency Assessment (AGHDA) scale.


Adverse effects include increases in weight, blood pressure, and blood glucose and lipid levels. These should be monitored together with plasma haemoglobin A1c (HbA1c).


In acromegaly, excess growth hormone causes diabetes, hypertension and arthritis. The former two lead to a two-fold excess in cardiovascular mortality. Surgery is the treatment of choice. Growth hormone secretion is reduced by octreotide, lanreotide and other somatostatin analogues, and to a lesser degree by bromocriptine and cabergoline. If surgery fails (nadir growth hormone during oral glucose tolerance test > 1 microgram/L) somatostatin analogues should be used. These bind to somatostatin receptors 2 and 5 to inhibit growth hormone production. About 60% of patients respond to somatostatin analogues.







Prolactin

is secreted by the lactotroph cells of the anterior pituitary gland. Its control is by tonic hypothalamic inhibition through dopamine, which in turn acts on D2 receptors of the lactotrophs. Its main physiological function is stimulation of lactation. Supra-physiological levels of prolactin inhibit gonadotrophin releasing hormone and gonadotrophin release as well as gonadal steroidogenesis.


Hyperprolactinaemia may be caused by drugs with antidopaminergic actions: antiemetics, major tranquillisers, second-generation neuroleptics, monoamine oxidase (MAO) inhibitors, tricyclic antidepressants and, to a lesser extent, oestrogens.


Hyperprolactinaemia may occur in primary hypothyroidism, in pituitary stalk disconnection or prolactin-secreting adenomas. Medical treatment is with bromocriptine started at 0.625 mg by mouth nightly, and titrated weekly to a maximum of 20 mg in divided doses. Cabergoline may be preferred as a more specific dopamine agonist than bromocriptine, which is taken once weekly, titrated from 500 micrograms to 2 mg. Higher doses (up to 6 mg weekly) are necessary only in the treatment of macroprolactinomas. Quinagolide is anther dopamine agonist; the dose is 25–150 micrograms at bedtime.


In pregnancy, the dopamine agonists are discontinued in microadenomas, where the risk of enlargement is small. Treatment should continue for macroadenomas because the risk of enlargement is much higher, 15–30%. Both bromocriptine and cabergoline are safe to use, although cabergoline is not licensed in pregnancy. Much higher doses of cabergoline (e.g. 4 mg daily or 28 mg weekly) have been associated with cardiac fibrosis, although this has not been reported in many groups of prolactinoma patients. Nevertheless, the UK regulatory agency (MHRA) advises cardiac valve monitoring for patients on any dose of cabergoline.


Trans-sphenoidal surgery in a specialist unit is an alternative to medical therapy in patients who do not tolerate, or are resistant to, dopamine agonists.




Posterior pituitary hormones and analogues





Desmopressin


Desmopressin (des-amino-D-arginine vasopressin, DDAVP) has two major advantages: the vasoconstrictor effect has been reduced to near insignificance and the duration of action with nasal instillation, spray or subcutaneous injection, is 8–20 h (t½ 75 min) so that, using it once to twice daily, patients are not inconvenienced by polyuria and nocturia.


Desmopressin is available as oral or sublingual tablets, nasal spray and injection. The adult dose for intranasal administration is 10–20 micrograms daily. The dose for children is about half that for adults. The bioavailability of intranasal DDAVP is 10%. It is also the only peptide for which an oral formulation is currently available, albeit with a bioavailability of only 1%. Tablets of DDAVP are prescribed initially at 200–600 micrograms daily in three divided doses. The main complication of DDAVP is hyponatraemia, which can be prevented by allowing the patient to develop some polyuria for a short period during each week. The dose requirement for DDAVP may decrease during intercurrent illness. It is therefore important to review the need for DDAVP daily in critically ill patients.


Nephrogenic diabetes insipidus, as is to be expected, does not respond to antidiuretic hormone.


In bleeding oesophageal varices, use is made of the vasoconstrictor effect of vasopressin (as terlipressin, a vasopressin prodrug); see page 564.


In haemophilia, desmopressin can enhance blood concentration of factor VIII.


Felypressin is used as a vasoconstrictor with local anaesthetics.


Enuresis: see page 313.



Diabetes insipidus: vasopressin deficiency


Diabetes insipidus (DI) is characterised by persistent production of excess dilute urine (> 40 mL/kg every 24 h in adults and > 100 mL/kg every 24 h in children). DI is classified as cranial or nephrogenic. Cranial causes of DI are genetic, developmental or idiopathic. Acquired causes are head injury, surgery to the hypothalamic–pituitary region, tumours, inflammatory conditions such as granulomatous and infectious disease, vascular causes and external radiotherapy. Nephrogenic DI has a larger number of causes including drugs (lithium, demeclocycline) and several diseases affecting the renal medulla. The DNA sequencing of the receptor and aquaporins has also allowed identification of mutations in these that cause congenital DI.






Sex (gonadal) hormones and antagonists: steroid hormones










Androgens


Testosterone is the predominant natural androgen secreted by the Leydig cells of the testis; in a normal adult male testosterone production amounts to 4–9 mg/24 h. It circulates highly bound to a hepatic glycoprotein called sex hormone binding globulin (65%) and loosely bound to albumin (33%). Only 1–2% of circulating testosterone is unbound and freely available to tissues. It is converted by hydroxylation to the active dihydrotestosterone (DHT). Testosterone is necessary for normal spermatogenesis, for the development of the male secondary sex characteristics, sexual potency and for the growth, at puberty, of the genital tract.


Protein anabolism is increased by androgens, i.e. androgens increase the proportion of protein laid down as tissue, especially muscle and (combined with training, increase strength). Growth of bone is promoted, but the rate of closure of the epiphyses is also hastened, causing short stature in cases of precocious puberty or of androgen overdose in the course of treating hypogonadal children.




Preparations and choice of androgens


Testosterone given orally is subject to extensive hepatic first-pass metabolism (see p. 86) and it is therefore usually given by other routes. Androgens are available for oral, buccal, transdermal or depot administration.









Antiandrogens (androgen antagonists)


Oestrogens and progestogens are physiological antagonists to androgens. But compounds that compete selectively for androgen receptors have been made.




Cyproterone


Cyproterone is a derivative of progesterone; its combination of structural similarities and differences results in the following:






Uses

Cyproterone is used for reducing male hypersexuality, and in prostatic cancer and severe female hirsutism. A formulation of cyproterone plus ethinylestradiol (Dianette, which contains only 2 mg of cyproterone acetate) is offered for this latter purpose as well as for severe acne in women; this preparation acts as an oral contraceptive but does not have a UK licence, and should not be used primarily for this purpose.


Flutamide and bicalutamide are non-steroidal antiandrogens available for use in conjunction with the gonadorelins (e.g. goserelin) in the treatment of prostatic carcinoma.


Finasteride and dutasteride (see p. 462), which inhibit conversion of testosterone to dihydrotestosterone, have localised antiandrogen activity in tissues where dihydrotestosterone is the principal androgen; they are therefore useful drugs in the treatment of benign prostatic hypertrophy.


Spironolactone (see p. 563) also has antiandrogen activity and may help hirsutism in women (as an incidental benefit to its diuretic effect). Androgen secretion may be diminished by continued use of a gonadorelin (LHRH) analogue (see p. 597).


Ketoconazole

Stay updated, free articles. Join our Telegram channel

Jun 18, 2016 | Posted by in PHARMACY | Comments Off on Hypothalamic, pituitary and sex hormones

Full access? Get Clinical Tree

Get Clinical Tree app for offline access