The responsible factors for IDD can be broadly classified in to two groups namely
Environmental factors: There are two important factors
Environmental iodine deficiency: Iodine deficiency is likely to occur in all elevated regions subject to higher rainfall with run-off in to rivers. High rainfall, snow and flooding increase the loss of soil iodine, which has often been already demanded by past glaciations. This causes the low iodine content of food for man and animals.
Iodine exists in nature in large quantities in sea water (50-60μg/l). sea water along with iodine evaporates to form clouds which condense in the form of rain and enrich the top layers of soil with iodine. The food crops that are grown on the soil and the animals while grazing on the plants assimilate iodine. Water from deep well can provide a major source of iodine.
Giotrogens: Giotrogens are certain chemical substances which interfere with iodine metabolism in the body. Thiocyanates, isothiocyanates, cyanogenic glycoside, flavanoids and thiourea are considered as goitrogens. These goitrogens are known to interfere with iodine metabolism at various levels. Some of the goitrogenic substances are found in abundance in certain tubers and vegetables like tapioca, cabbage and cauliflower. Common foods like sorghum, ragi, mustard and groundnut are known to contain a fair amount of goitrogens.
Intrinsic factors: Failure to synthesize the thyroid harmone due to inherited and congenital defects in the hormone synthesis and secretion.