Impact of habitual seaweed consumption on iodine nutrition and thyroid function: a non-randomized pre-post clinical study

Macroalgae, also called seaweed, may serve as a dietary iodine source, although some species contain high iodine levels and may increase the risk of excessive intake. This non-randomized pre-post clinical trial aimed to assess iodine status, estimated iodine intake and thyroid function in habitual seaweed consumers (pre-intervention), and after a six-week cessation period of seaweed consumption (post-intervention). Habitual seaweed consumers (n = 49) were recruited by convenience sampling from the two largest cities in Norway. Urinary iodine concentration (UIC), urinary creatinine, iodine intake and thyroid function were assessed pre- and post-intervention. The thyroid function was assessed biochemically by measuring TSH, fT3, fT4 and TPO antibodies in serum. Seaweed intake was assessed using a seaweed specific food frequency questionnaire (FFQ). The median (p25-p75) UIC and estimated iodine intake was 270 (185–970) µg/L and 658 (330–1516) µg/ day at pre-intervention and decreased substantially to 87 (52–138) µg/L and 189 (142–264) µg/ day post-intervention. Median estimated iodine intake pre-intervention exceeded the Tolerable Upper Intake Level (UL) for iodine, whereas post-intervention the estimated iodine intake was substantially decreased to well below the UL. The median serum TSH decreased significantly after cessation of seaweed consumption (from pre- to post-intervention). The largest decrease in TSH was seen in participants with the highest estimated iodine intakes pre-intervention. Ingestion of seaweed with high iodine content could potentially pose a risk of exceeding the tolerable upper intake level for iodine. Cessation of seaweed resulted in a significant decrease in TSH in this study.