Iodine and Iodides

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Chapter: Essential pharmacology : Thyroid Hormones And Thyroid Inhibitors

Though iodine is a constituent of thyroid hormones, it is the fastest acting thyroid inhibitor. It is reduced in the intestines to iodide and the response to iodine or iodides is identical. The gland, if enlarged, shrinks, becomes firm and less vascular.


IODINE AND IODIDES

 

Though iodine is a constituent of thyroid hormones, it is the fastest acting thyroid inhibitor. It is reduced in the intestines to iodide and the response to iodine or iodides is identical. The gland, if enlarged, shrinks, becomes firm and less vascular. The thyroid status starts returning to normal at a rate commensurate with complete stoppage of hormone release from the gland. The gland itself involutes and colloid is restored. With daily administration, peak effects are seen in 10–15 days, after which ‘thyroid escape’ occurs and thyrotoxicosis may return with greater vengeance. Worsening of hyperthyroidism especially occurs in multinodular goiter.

 

All facets of thyroid function seem to be affected, but the most important action is inhibition of hormone release—‘thyroid constipation’. Endocytosis of colloid and proteolysis of thyroglobulin comes to a halt. The mechanism of action is not clear. It appears to be a direct action on thyroid cells, though attenuation of TSH and cAMP induced thyroid stimulation has been demonstrated. Excess iodide inhibits its own transport in thyroid cells and may alter the redox potential of cells, thus interfering with iodination  reduced T3/T4 synthesis (WolffChaikoff effect).

 

Preparations And Dose

 

Lugol’s solution (5% iodine in 10% Pot. iodide solution): LUGOL’S SOLUTION, COLLOID IODINE 10%: 5–10 drops/day. COLLOSOL 8 mg iodine/5 ml liq.

 

Iodide (Sod./Pot.) 100–300 mg/day (therapeutic), 5–10 mg/ day (prophylactic) for endemic goiter.

 

Uses

 

1. Preoperative preparation for thyroidectomy: generally given for 10 days just preceding surgery. The aim is to make the gland firm, less vascular and easier to operate on. Though iodide itself will lower the thyroid status, it cannot be relied upon to attain euthyroidism which is done by use of carbimazole before starting iodide. Propranolol may be given additionally for rapid control of symptoms.

 

2. Thyroid storm Lugol’s iodine (6–10 drops) or iodine containing radiocontrast media (iopanoic acid/ipodate) orally are used to stop any further release of T3/T4 from the thyroid and to decrease T4 to T3 conversion.

 

3. Prophylaxis of endemic goiter It is generally used as “iodized salt”.

 

4. Antiseptic  As tincture iodine, etc. see Ch. No. 65.

 

Adverse Effects

 

1.  Acute reaction It occurs in sensitive individuals only—swelling of lips, eyelids, angioedema of larynx (may be dangerous), fever, joint pain, petechial haemorrhages, thrombocytopenia, lymphadenopathy.

 

2. Chronic overdose (iodism) Inflammation of mucous membranes, salivation, rhinorrhoea, sneezing, lacrimation, swelling of eyelids, burning sensation in mouth, headache, rashes, g.i. symptoms, etc. The symptoms regress on stopping iodide ingestion.

 

Longterm use of high doses can cause hypothyroidism and goiter.

 

Iodide may cause flaring of acne in adolescents. Given to pregnant or nursing mothers, it may be responsible for foetal/infantile goiter and hypothyroidism.

 

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