Pharmacology for Anaesthesia and Intensive Care

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Section IVOther important drugs

Iodides

Iodide has been given as potassium iodide orally, sodium iodide intravenously and

iodide containing radiographic contrast dyes. It is concentrated in the follicular cells

where peroxidases oxidize it to iodine, which is then combined with tyrosine resi-

dues.

Iodide is required for normal thyroid function and when levels are too low or too

high, thyroid function becomes abnormal. Large doses cause inhibition of iodide

binding and hence reduced hormone production. This effect is greatest when iodide

transport is increased, that is, in thyrotoxicosis. Iodide also reduces the effect of TSH

on the thyroid gland and inhibits proteolysis of thyroglobulin. Thyroid vascularity

is decreased, which is useful pre-operatively, but iodide is not used for prolonged

periods as its actions tend to diminish with time.

Lithium may be considered in those with iodide sensitivity.

Oral contraceptives

The oral contraceptive pill (OCP) takes two forms: a combined oestrogen/

progesterone preparation and a progesterone-only preparation. The former is taken

for 21 consecutive days followed by a 7 day break, while the latter is taken continu-

ously.

Mechanism of action

The combined preparation inhibits ovulation by suppressing gonadotrophin-

releasing hormone and inhibiting gonadotrophin secretion. The progesterone-only

pill prevents ovulation in only 25% of women and appears to work by producing an

unfavourable endometrium for implantation.

Side effects


Major – these include cholestatic jaundice and an increased incidence of thrombo-

embolic disease. The following are considered a contraindication for the combined

OCP: a history of thrombo-embolism, obesity, smoking, hypertension or diabetes

mellitus.


Minor – these include breakthrough bleeding, weight gain, breast tenderness,

headache and nausea. Hirsutism and depression may occur following the com-

bined OCP. Side effects are lower in the progesterone-only pill.

Hormone replacement therapy

Hormone replacement therapy (HRT; oestrogen or oestrogen/progesterone combi-

nations) is used to relieve menopausal symptoms, reduce osteoporosis and reduce

the risk of cardiovascular disease.