Iodized salt is considered as the most appropriate measure for iodine supplementation (2, 4, 62, 145, 146).

The advantage of supplementing with iodized salt is that it is used by all sections of a community irrespective of social and economic status. It is consumed as a condiment at roughly the same level throughout the year. Its production is often confined to a few centers which means that processing can occur on a larger scale and with better controlled conditions. However, this is often not the case in developing countries.

There are two forms of iodine which can be used to iodise salt: "iodide" and "iodate" usually as the potassium salt. Iodate is less soluble and more stable than iodide and is therefore preferred for tropical moist conditions. Both are generally referred to as "iodised" salt.

The daily requirement of iodine is 150 ug per person for adults (2). The level of iodination of salt has to be sufficient to cover this requirement together with losses from the point of production to the point of consumption including the expected shelf life. It also has to take into account the per capita salt consumption in an area. Previously, generally accepted levels of salt consumption in the range 10-15 g per day are now regarded as excessive because of the increased liability to hypertension. For this reason, levels in the range of 3-6 g per day are being recommended in Western countries. Iodinated salt will also be needed as a feed supplement for cattle and other livestock in iodine deficient areas. Allowing for these factors, the level of iodine as iodate being used at present to provide 150 ug of iodine by day is in the range of 20-40 mg per kg (147).

The packaging of the iodised salt is very important. Jute bags have been used extensively but in humid conditions, the salt absorbs moisture. The iodate dissolves and will drip out of the bag if it is porous, with a heavy loss. This has been found to reach 75% over a period of nine months. To avoid this waterproofing is required, achieved by a polythene lining inside the jute bag or else a plastic bag. The additional cost of a plastic bag (50-80% more) would be justified by reduced losses and their resale value (145, 146).

The other aspects of the use of iodized salt in the prevention of IDD have been extensively discussed by Mannar and Dunn (145) : the control of the iodine concentration in salt at production level should be performed by using titration methods or, in the case of imported salt, by using reliable test kits at the point of entry. Consignments with suspect iodine levels should be rechecked by titration. National monitoring programs should include monitoring sentinel sites to carry out the following activities : monitoring periodically salt iodine levels in retail shops and households using reliable test kits, conducting occasionally goiter prevalence surveys and measuring regularly urinary iodine. In order to determine the proportion of households using adequately iodized salt in a large geographic area, it is recommended to use cluster surveys at the provincial or national levels. It is also recommended to identity high risk communities, or « hot spots » where there is an inadequate proportion of households using adequately iodized salt.

Iodised oil ("lipiodol") was first used for the correction of iodine deficiency in Papua New Guinea (60). In a controlled trial in the Boana area of the Huon Peninsula of New Guinea, McCullagh carried out a double blind follow-up over 3 years which revealed successful prevention of goiter (148). In subsequent laboratory studies on the same population, Buttfield and Hetzel (149) demonstrated both severe iodine deficiency and the effectiveness of the single iodised oil injection (4 ml) in correcting iodine deficiency for a period of up to 4 1/2 years (Table 7). A further controlled trial in the Western Highlands of New guinea revealed the prevention of endemic cretinism provided the injection was given before pregnancy and a reduction in recorded fetal and neonatal deaths in the treated group (112).

A further advantage has been the subsidence of established goiter within one to three months of the injection. (Fig. 7) This is much appreciated by the goitrous subjects. When coupled with increased energy and well being consequent on the correction of hypothyroidism, there has been continued demand for the measure, originally in Papua New Guinea, and in many other countries since.

Extensive additional studies on the use of iodized oil in the correction and prevention of IDD have subsequently be conducted in Latin America, Africa, Asia and Eastern Europe (review in ref. 150).

The physiology and pharmacology of iodized oil in goiter prophylaxis has recently been extensively reviewed (151).

In excess of 20 million injections of iodised oil have been given since 1974 with very little in the way of side effects apart from a rare abscess at the site of injection. Refrigeration is not required, which is a great advantage. Iodised oil is certainly an effective means for the correction of iodine deficiency and has opened up the possibility of elimination of IDD as a public health problem in the next decade. However the necessity for an injection has been questioned, in view of the costs of the syringe and needles and the necessity to have specially trained staff to give the injections. If the staff are readily available through the primary health care system, then the costs are comparable to those of iodated salt: 5-10 US cents per person per year. On the other hand, if the oil can be given orally it would be possible to use village health volunteers to supervise the administration of the oil. This would make it much more readily available to village communities with severe IDD problems. Another advantage of the oral preparation is the freedom from the risk of AIDS or hepatitis B infection from contaminated syringes, although this should be eliminated by proper sterilisation of needles or by using disposable syringes. Recent experience has confirmed the convenience of the oral administration of iodised oil at yearly intervals through the primary health care system at a village level. In general the effect of oral administration lasts half the time of the same dose given by injection (7, 151, 152).

As the populations covered increased, the injection dosage has been reduced. The latest recommendations by WHO-UNICEF-ICCIDD is shown in Table 8. Even lower doses have been recommended by some authors (153, 154).

Iodised bread was used in Tasmania in preference to both iodised salt and iodide tablets distributed through the schools, and shown to be effective (158). Its use was discontinued because of the availablility of other sources of iodine, notably from milk consequent to the use of iodophors in the dairy industry. It is for this reason that milk has become a major adventitious source of iodine in many Western countries such as the USA, the United Kingdom, and in Northern Europe. A change in dairy practice would reverse the situation and increase the likelihood of iodine deficiency in the population. Successful use of iodized bread was also reported from Russia when bread become a staple (159).

Iodised water. Water has some of the advantages of salt as a vehicle for iodine fortification. Both are daily necessities and thus their iodization will reach the most vulnenrable groups – the poor, the isolated. Water fortified at a regular rate with iodine provides the thyroid with a steadily daily ration, which is physiologically desirable (160).

Systems for iodization of drinking water can be classified as follows :

a. Silicone elastomers releasing iodine. A commercial version of this approach is the “Rhodifuse” system of Rhône-Poulenc-Rorer-Doman (now Adventis). Silicone matrices containing 30 % sodium iodide are placed in polyethylene baskets. When the baskets are placed in wells, sodium iodide is released into the water according to the porosity and the surface/volume ratio. Initially successfully used in Mali (161), the system was then used in Burkina Fasso and the Central African Republic (150 ). Limitations to the system were climatic conditions, the cost of the diffuser and its maintainance and occasional difficulties between the craftsmen and the populations. A similar device was manufactured locally in Malaysia (162).

b. Iodide added to running water in pipes. A commercial example is the Hydroline system used in Sicily (163). The apparatus consisted of a canister filled with coarse crystals of iodine through which water was diverted from the line by a pressure differential. The system was highly efficient with a cost estimated at $ 0.04/person/day. A similar system used in the Sarawak region of Malaysia resulted in the reduction of goiter rate from 61 % to 30 % within 9 months together with significant rises in T4 and falls in TSH (164).

c. Iodide added to run-off water. This technique was used by Cao at all (165) in Southern Xinjiang ,China. It resulted in increase of iodine intake by plants, possible benefits on rice production, increased iodine uptake by thyroid in sheeps, chickens and eggs and increased survival of newborn lambs.

d. Manual addition of iodine to standing water. In this approach, iodine is added directly to vessels containing drinking water. It has been used most notably in northern Thailand.

ICCIDD concluded from a world review of water iodization programs (160) that when properly monitored, the procedure is efficient in controlling iodine deficiency but that it is generally more expensive than iodized salt in large-scale national programs and that it is unlikely to be self-sustaining in poor rural countries and thus requires permanent external funding.