Use of iodine: thyroid consequences
Solution of Lugol
Iodine is undoubtedly the oldest remedy used in thyroid affections. It is generally used as aqueous iodine oral solution, often called Lugol solution. One milliliter of this solution contains 130 milligrams of total iodine, about 6 milligrams per drop. This preparation supplies a quantity of iodine without comparison with the normal daily intake which is about 100 to 200 micrograms
The mechanism of action of iodine at extraphysiological dose is complex. Its principal effect is to inhibit the proteolytic enzymes (catheptases) which release thyroid hormones from thyroglobulin. This inhibition is usually transitory. Lugol solution given to patients with hyperthyroidism causes initially, during about fifteen days, a clear improvement of the symptoms, but, thereafter, its beneficial effect attenuates and the disease takes again its course, sometimes with a higher intensity.
In practice, Lugol solution is only used in particularsituations:
- preoperative treatment before thyroidectomy: to prevent thyrotoxis crisis and to reduce the vascularity of the gland to facilitate the surgical intervention.
- emergency treatment of thyrotoxic crisis where the iodine can be given by intravenous route.
- treatment of edematous exophtalmy in patients with severe Graves' disease.
It should be noticed that a preliminary treatment withiodine could delay the action of antithyroid drugs given later and impair the use of radioactive iodine.
The intake of an excessive quantity of iodine can induce in normal subjects either hypothyroidism, or hyperthyroidism. It can also cause symptoms of iodism such as hypersalivation with metallic taste in the mouth, rhinitis, conjunctival tearing, edema. Allergicevents, pruritus, urticaria, were observed, especially with iodized preparations intended for radiological exploration.
The contra-indications of Lugol solution are pregnancy and allergy to iodine.
Radioactive iodine
The stable isotope of iodine has mass 127 and there exist several radioactive isotopes among which iodine 123 whose half-life (radioactive and not pharmacocinetic) is 12 hours and iodine 131 whose half-life is 8 days.
Radioactive iodine like stable iodine has a high affinity for the thyroid in which it accumulates.
Its therapeutic uses are either diagnostic, ? scanning of the iodine binding of the thyroid, or therapeutic, ? treatment of certain hyperthyroidisms and post-operative treatment of thyroid cancers.
Radioactivity emitted by radioactive iodine used at therapeutic doses in patients with hyperthyroidism causes a partial destruction of the thyroid with progressive clinical improvement over 2 or 3 months. The principal interest of radioactive iodine lies in the simplicity of administration, avoiding, particularly in elderly, antithyroid drugs or thyroidectomy.
Used in post-operative treatment of thyroid cancer, radioactive iodine kills the possible remaining malignant cells.
Iodized drugs
Besides Lugol solution, various drugs contain one or more atoms of iodine. It is the case of amiodarone which, on prolonged therapy, supplies a great quantity of iodine and is often at the origin of thyroid disorders, in general hyperthyroidism, sometimes hypothyroidism.
In addition, it is necessary to avoid iodine poisoning, in the new-born baby in particular, by using iodized disinfectants which are absorbed by the skin or the mucous membranes.
The pharmacology of the iodized products, apart from their interaction with the thyroid, is studied separately
Remarks
- Beta-blockers
Beta-blockers, in particular propranolol, are usually used in the treatment of hyperthyroidism, especially at the beginning of treatment. Propranolol decreases very quickly palpitations, tachycardia, nervousness, sweats, tremor, without effects on tiredness and loss of weight of hyperthyroidism.
- Corticoids
High corticoid doses are sometimes used in patients with severe hyperthyroidism before surgical operations for example.
- Surgery
Subtotal thyroidectomy, which removes approximately 9/10th gland, obtains excellent results in the treatment of Graves' disease, with relapses much fewer than with treatment with the antithyroid drugs.
|
