Iodine deficiency has historically been considered an issue for developing countries rather than industrialised countries such as the United Kingdom (UK). Endemic thyroid swelling (goitre) associated with iodine deficiency was at one time widespread in the UK and Medical Research Council surveys in 1924 and 1944 reported visible goitre in up to 50% of adult women and schoolgirls. No salt iodisation programme was adopted in the UK unlike in other European countries and iodised salt is not available to buy in the UK.

 

Since the 1940s significant changes in farming practice in the UK were associated with a rise in the iodine content of milk, particularly during winter months when cattle are dependent on iodine rich artificial feed. In addition, successive UK governments from the 1940s encouraged increased milk consumption in schoolchildren. By the 1980s this resulted in the iodine content of milk alone being almost sufficient to meet the recommended daily requirement of 150 µg per day and has been described as an “accidental public health triumph”. Although national monitoring of milk iodine content continued the major concern was to avoid the harmful effects of iodine toxicity rather than deficiency.

 

Recently concern had also been expressed about the UK iodine status as up to 50% of UK women of child-bearing age screened in small studies at single centres were iodine deficient. A similar pattern of iodine deficiency has been seen in Ireland, with iodine intake being particularly low in the summer months. 

A survey of the UK iodine status was performed in 2009 by the British Thyroid Association, and supported by the British Thyroid Foundation. This found mild iodine deficiency in a sample of 737 schoolgirls. The study focused on young female subjects aged 14-15 years who are pre-pregnancy as in the short to medium term it is their children that are the most susceptible to the adverse effects of iodine deficiency. Milk intake was positively associated with higher urine iodine concentration. As milk iodine concentrations were unchanged it was likely to be the reduced amount of milk now drunk by the population which is responsible for the decline in iodine status.

 

Subsequently a study using samples and data from the Avon Longitudinal Study of Parents and Children (ALSPAC) related maternal iodine deficiency in pregnancy from stored urine iodine samples to cognitive outcomes such as IQ and reading ability in offspring up to the age of nine years. This study had recruited pregnant women in the early 1990s from the South West of the UK and collected data during pregnancy and followed the health and development of their children. The women were classified as being iodine deficient and children born to mothers broadly classified as iodine deficient were more likely to have lower verbal IQ scores and reading accuracy and reading comprehension scores.

 

In summary there is increasing evidence that iodine deficiency may also now be an emerging issue in industrialised countries such as the UK, previously thought of as iodine-sufficient.

The role of iodine

Iodine is an essential component of the thyroid hormones thyroxine (T4) and tri-iodothyronine (T3) produced by the thyroid gland which play a crucial role in brain and neurological development. The ideal dietary allowance of iodine recommended by World Health Organisation (WHO) is 150μg of iodine per day which increases to 200-250µg per day in pregnancy. Iodine deficiency is recognised as a global problem with large populations at risk who are living in an environment where the soil has been deprived of iodine. This arises from past glaciation, compounded by the leaching effects of snow, water and heavy rainfall, which removes iodine from the soil. The mountainous regions of Europe, the Northern Indian Subcontinent, the extensive mountain ranges of China, the Andean region in South America and the lesser ranges of Africa are all iodine deficient. In 1980 the WHO estimated that at least one-third of the world’s population was iodine deficient, predominantly those living in developing countries.

 

Severe iodine deficiency may be associated with impairment in the psycho-neurological outcome in children because both mother and offspring are exposed to iodine deficiency during and after the pregnancy. Iodine supplementation was shown to eliminate new cases of severe neurological impairment in children, reduce infant mortality and improve intellectual function in the general population. Even mild iodine deficiency is thought to lead to reductions of 10-15 in IQ points.

 

Iodine deficiency remains a huge problem and many people are still deficient in iodine, despite major national and international efforts to increase iodine intake, primarily through the voluntary or mandatory iodisation of salt. The WHO estimates that two billion people, including 285 million school-age children, still have iodine deficiency. International efforts to control iodine deficiency are slowing, and reaching the third of the worldwide population that remains deficient poses major challenges.

 

Iodine deficiency disorders

The main change in thyroid function associated with the pregnant state is the requirement for an increased production of thyroid hormone that depends directly upon the adequate availability of dietary iodine and the underlying integrity of the thyroid gland. Physiological adaptation can take place when the iodine intake is adequate. When iodine intake is deficient, pregnancy can lead to the development of a thyroid swelling (called a goitre) and lower thyroid hormone concentrations.

 

The term iodine deficiency disorders (IDD) refers to all the ill-effects of iodine deficiency in a population that can be prevented by insuring that the population has an adequate intake of iodine. Severe iodine deficiency results in severe learning disabilities, deafness and impaired motor development. The effects of mild-to-moderate iodine deficiency on cognition are less well known than those of moderate-to-severe deficiency but it is assumed that there is a continuum of disability with more subtle impairments of IQ and motor ability associated with less severe deficiency.

 

The spectrum of iodine deficiency disorders:

Fetus:

• Abortions
• Stillbirths
• Congenital anomalies
• Increased perinatal mortality
• Endemic severe neurological impairment

 

Neonate                                

• Neonatal goitre
• Neonatal hypothyroidism
• Endemic mental retardation
• Increased susceptibility of the thyroid to nuclear radiation

 

Child and adolescent         

• Goitre
• (Subclinical) hypothyroidism
• Impaired mental function
• Retarded physical development
• Increased susceptibility of the thyroid to nuclear radiation

 

Adult                                      

• Goitre with its complications
• Hypothyroidism
• Impaired mental function
• Spontaneous hyperthyroidism in the elderly
• Iodine-induced hyperthyroidism
• Increased susceptibility of the thyroid to nuclear radiation

 

Mild-to-moderate iodine deficiency occurs in areas that are not immediately recognised as iodine deficient. Although the introduction of iodised salt has considerably improved the situation globally in the developing world, iodine deficiency remains an issue in continental Europe where it is estimated that up to 50% of children live in iodine deficient communities. As maternal thyroid hormone is crucial to fetal nervous system maturation, even modest states of iodine deficiency could be deleterious. Data suggest that the children of women with hypothyroxinaemia may have psycho-neurological deficits and delayed mental and motor function. There are few data of the consequences of mild-to-moderate iodine deficiency in older children. Iodine supplementation may improve perceptual reasoning suggesting that mild iodine deficiency may prevent children attaining their full intellectual potential.

• Unfortunately there is no reliable method for assessing iodine status in an individual. 
• Urinary iodine concentration cannot be used to assess iodine status of an individual. This is because the concentration is affected by urine volume produced (iodine concentration may be low simply because the urine is dilute). In addition, iodine excretion varies day-to-day and therefore more than one urine sample (at least 10) would be required to assess usual status.
• A brief dietary assessment can reveal those who are not frequent consumers of iodine-rich foods (e.g. milk, dairy products, white fish, and eggs)  and should ring warning bells that the individual may be at risk of iodine deficiency. 

In the UK the risk groups are:

• Pregnant women (as the recommended intake is higher than the non-pregnant recommendation)
• Anyone who completely avoids milk and dairy products (or consumes them in limited quantities)
• Anyone who avoids fish or consumes them in limited quantities
• Vegans are a recognised at-risk group for iodine deficiency
• Vegetarians may also have a low iodine intake depending on the amount of milk and dairy products in their diet. Many dairy-milk substitutes (e.g. soya) are not enriched with iodine.