Iodine Intake and the Pattern of Thyroid Disorders: A Comparative Epidemiological Study of Thyroid Abnormalities in the Elderly in Iceland and in Jutland, Denmark

Abstract

Thyroid abnormalities are common in all populations, but it is difficult to compare results of epidemiological studies, because different methods have been used for evaluation. We studied the importance of the population iodine intake level for the prevalence rate of various thyroid abnormalities in elderly subjects. Random samples of elderly subjects (68 yr) were selected from the central person registers in Jutland, Denmark, with low (n = 423) and, in Iceland, with longstanding relatively high (n = 100) iodine intake. Females from Jutland had a high prevalence of goiter or previous goiter surgery (12.2%), compared with males from Jutland (3.2%) and females (1.9%) and males (2.2%) from Iceland. Abnormal thyroid function was very common in both areas, with serum TSH outside the reference range in 13.5% of subjects from Jutland and 19% of those from Iceland. In Jutland, it was mainly thyroid hyperfunction (9.7% had low, 3.8% had high serum TSH), whereas in Iceland, it was impaired thyroid function (1% had low, 18% had high serum TSH). All subjects with serum TSH more than 10 mU/L had autoantibodies in serum, but antibodies were, in general, more common in Jutland than in Iceland. Thus, thyroid abnormalities in populations with low iodine intake and those with high iodine intake develop in opposite directions: goiter and thyroid hyperfunction when iodine intake is relatively low, and impaired thyroid function when iodine intake is relatively high. Probably, mild iodine deficiency partly protects against autoimmune thyroid disease. Thyroid autoantibodies may be markers of an autoimmune process in the thyroid or secondary to the development of goiter.

Iodine status of pregnant women in a population changing from high to lower fish and milk consumption

“…For many decades Iceland has been known for its high iodine status (5)(6)(7)23) . With the sharp decreases in milk and fish intakes during past decades, worries about iodine status in the population have increased (11,12,24) .…”

“…All European countries except Iceland have experienced this health and socio-economic threat to a greater or lesser extent (4) . The Icelandic population has in the past been known for its high iodine status, based on studies from 1939, 1988 and 1998 (5)(6)(7) . The good iodine status observed in these studies was suggested to be due to high fish consumption and abundant milk, both important dietary sources of iodine (8)(9)(10) .…”

Impact of iodination on thyroid pathology in Africa

“…15 Iodine deficiency disorders abound in areas with inadequate iodine intake while autoimmune thyroid disorders are rare in iodine deficiency but become prevalent with transition to iodine sufficiency. 16 Monitoring disease patterns in Africa has proved challenging in the absence of 397…”

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“…Another survey in Italy found an overall prevalence of thyroid functional autonomy (i. e., subnormal TSH concentrations and normal concentration of FT3 and FT4) of 4.6 %. Finally, the overall prevalence of subclinical hyperthyroidism was lower in Spain (1.3 %) [24] and higher in Denmark (9.7 %) [25].…”