Metabolic disorders

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Chapter 37 Metabolic disorders

Chapter contents

Pineal gland 287
Pituitary gland 287
Thyroid gland 288
Parathyroid gland 289
Adrenal glands 291
Thymus gland 292
Reproductive hormones 292
Pancreas 292
Other metabolic conditions 294

Pineal Gland

Synthesizes melatonin, which – because of its position in the third ventricle – is secreted directly into the cerebrospinal fluid (CSF), from where it finally ends up in the blood. Melatonin affects reproductive development and daily physiology.

Pituitary Gland

The ‘master gland’ of the body; possesses two lobes:

• Anterior (adenohypophysis): is attached but not directly connected to the hypothalamus.
• Posterior (neurohypophysis): directly connected to the hypothalamus via neural tissue. Does not produce its own hormones but stores and releases hormones when they are required.

Anterior Pituitary Gland

The anterior pituitary gland secretes:

• Growth hormone (GH): stimulates growth of bones, muscles, etc. by stimulating protein synthesis.
• Follicle-stimulating hormone (FSH).
• Prolactin (PL): stimulates the development of female breast glandular tissue during pregnancy and milk production after birth.
• Luteinizing hormone (LH).
• Thyroid-stimulating hormone (TSH): stimulates secretion of thyroid gland.
• Adrenocorticotrophic hormone (ACTH): stimulates secretion of adrenal glands.
• Endorphins.

The anterior pituitary gland responds to:

• Growth-hormone-releasing hormone (GHRH): causes the release of growth hormone (GH), which controls growth.
• Thyrotropin-releasing hormone (TRH): causes the release of thyroid-stimulating hormone (TSH) and prolactin (PL).
• Corticotrophin-releasing hormone (CRH): causes the release of adrenocorticotrophic hormone (ACTH), which controls blood pressure.
• Dopamine (DA): inhibits release of prolactin under normal conditions.
• Gonadotrophin-releasing hormone (GnRH): causes release of FSH and LH, which control pregnancy.

Posterior Pituitary Gland

The posterior pituitary gland secretes:

• Oxytocin: involved in childbirth.
• Antidiuretic hormone (ADH): affects blood pressure, water and concentration of salts in the nephron (see Chapter 26 ‘Cardiovascular disorders’, p. 193).

Treatment of Pituitary Disorders

It is evident from the above that any patient with a pituitary condition of any kind will probably be under orthodox medical care. This can be complex and the patient is carefully monitored. However, it is worth being aware of the condition of diabetes insipidus, which is caused by a reduction in ADH. The patient will urinate excessively. The difference between this and diabetes mellitus is the normal blood sugar levels.

A careful case history should enable a practitioner to identify the medication the patient is on and consult a formulary for the actions of the drug and any side effects. Note that severe trauma to the head can be responsible for pituitary dysfunction.

Thyroid Gland (see Chapter 41 ‘scientific tests’ Table 41.1, p. 340, Table 41.2, p. 341)

Dysfunction of this gland is commonly encountered in clinical practice.

The thyroid is a double-lobed endocrine gland situated in the neck. It produces the hormones thyroxine (T4) and triiodothyronine (T3). These hormones are responsible for:

• regulating metabolic rate generally
• affecting growth.

Iodine is necessary for the production of both hormones. Disorders of the thyroid include:

• Hyperthyroidism: overactive thyroid
• Hypothyroidism: underactive thyroid.

Thyroid-releasing hormone is released by the hypothalamus, via the pituitary gland (see above); occasionally the pituitary is at fault.

Hyperthyroidsm

• Symptoms

• Palpitations, tachycardia.
• Agitation.
• Insomnia.
• Weight loss.

• Symptomatic Relief

Non-selective beta-blockers, 235 because tremor usually responds to beta-2 and not beta-1-blockers (see Chapter 31 ‘The nervous system’, p. 235).

• Side Effects

See Chapter 31 ‘The nervous system’ (p. 235).

• Treatment

• Radioactive iodine is used to destroy some of the thyroid cell, thus reducing production. Very often, however, this causes permanent hypothyroidism and the appropriate medication has to be given.
• Drugs that interfere with the synthesis of thyroid hormone, e.g. methimazole (US), carbimazole (UK).

Hypothyroidism

• Symptoms

• Fatigue.
• Weakness.
• Weight gain.
• Coarse dry hair, hair loss and dry rough pale skin.
• Muscle cramps.
• Depression and irritability.
• Abnormal menstrual cycles.
• Decreased libido.

Only severe cases have goitre.

• Treatment

• Thyroid replacement therapy: thyroxine (T4) is normally converted to triiodothyronine (T3). It is thyroxine that is normally used in treatment.

Adverse Effects

• Symptoms of hyperthyroidism.
• Disease related to calcium metabolism.

Parathyroid Gland

Calcium Regulation in the Body

• The amount of calcium in the blood and fluid has to be tightly controlled. There is always some loss of calcium from the body, (100–240 mg in the urine and between 45 and 100 mg in the faeces) but this is not a problem if it is being replaced.
• The parathyroid glands detect when the calcium concentration in the blood decreases and secrete parathyroid hormone (PTH). PTH stimulates the conversion of inactive vitamin D to active calcitriol.
• Calcitriol increases the absorption of calcium from the small intestine.
• The PTH and calcitriol stimulate the release of calcium from bone by activating osteoclasts (responsible for bone resorption) and increase the absorption of calcium in the kidneys.
• An increase in the concentration of the calcium in the blood above normal levels will switch off the production of PTH. The kidneys will then begin to excrete excess calcium in the urine.

Although this system maintains good control of calcium in the body, it does so at the expense of the calcium in the bone.

Interestingly, the efficiency of absorption of calcium from a supplement is inversely proportional to the total amount of calcium ingested. The cells of the gut wall have a maximum capacity for how much calcium they can pass across from the lumen to the capillaries.

Vitamin D is required for optimal calcium absorption.

image

Figure 37.1 Demonstrating the interrelationship between calcium and vitamin D and calcium metabolism.

Interaction of Food Substances with Calcium

See Chapter 13 ‘Vitamins and minerals’ (p. 96).

Osteoporosis

Bone continually remodels itself. This occurs due to action of the osteoclasts, which dissolve and reabsorb bone, and the osteoblasts, which synthesize new bone to replace that reabsorbed.

Osteoporosis can occur when the rate of bone resorption is greater than its formation. Although osteoporosis is thought to be a postmenopausal problem it is possible even in children. Osteoporosis is caused by more than one factor. Risk factors are:

• Female sex.
• Oestrogen deficiency.
• Smoking.
• Metabolic disease such as hyperthyroidism.
• Certain medication such as corticosteroids and anticonvulsants.

There is also an element of genetic and lifestyle factors.

• Assessment of Osteoporosis or Osteopaenia

• Full blood counts and hormone levels are checked for underlying causes.
• Alkaline phophatase levels (see Chapter 41 ‘Scientific tests’, p. 335) are usually raised.
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