An increased incidence of overt hypothyroidism after iodine fortification of salt in Denmark: a prospective population study.
BACKGROUND
Iodine fortification (IF) of salt was introduced in Denmark in 1998. Little is known about the effect of a minor increase in iodine intake on the incidence of hypothyroidism. We prospectively identified all new cases of overt hypothyroidism in two areas of Denmark before and for the first 7 yr after IF had been introduced.


METHODS
A computer-based register was used to identify continuously all new cases of overt hypothyroidism in two subcohorts with previous moderate and mild iodine deficiency (ID), respectively (Aalborg, n = 310,124, urinary iodine = 45 microg/liter; and Copenhagen, n = 225,707, urinary iodine = 61 microg/liter). Data were obtained 1) before IF (1997-1998), 2) during voluntary IF (1999-2000), 3) during early (2001-2002) and 4) during late (2003-2005) period with mandatory IF.


RESULTS
The overall incidence rate of hypothyroidism increased during the study period: baseline, 38.3/100,000.yr; voluntary IF, 43.7 (not significant vs. baseline); early mandatory IF, 48.7 [vs. baseline, rate ratio (RR) = 1.27; 95% confidence interval (CI) = 1.10-1.47]; and late mandatory IF, 47.2 (vs. baseline, RR = 1.23; 95% CI = 1.07-1.42). There was a geographic difference because hypothyroidism increased only in the area with previous moderate ID: Aalborg, late mandatory IF vs. baseline, 40.3/29.7 (RR = 1.11; 95% CI = 1.11-1.66); Copenhagen, 56.7/51.6 (RR = 1.10; 95% CI = 0.90-1.34). The increase occurred in young and middle-aged adults.


CONCLUSIONS
Even a cautious iodization of salt was accompanied by a moderate increase in the incidence rate of overt hypothyroidism. This occurred primarily in young and middle-aged subjects with previous moderate ID.
Pedersen
Inge Bülow
IB
Department of Endocrinology and Medicine, Aalborg Hospital, Aarhus University Hospital, DK-9000 Aalborg, Denmark. I.Bulow@rn.dk
Laurberg
Peter
P
Knudsen
Nils
N
Jørgensen
Torben
T
Perrild
Hans
H
Ovesen
Lars
L
Rasmussen
Lone Banke
LB
D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007037:Hypothyroidism / Q000453:epidemiology*; D007223:Infant; D007455:Iodine / Q000008:administration & dosage*; D008297:Male; D008875:Middle Aged; D011153:Population; D011446:Prospective Studies; D017673:Sodium Chloride, Dietary; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood
D017673:Sodium Chloride, Dietary; D013972:Thyrotropin; D007455:Iodine; D013974:Thyroxine