Iodine is essential for the production of thyroid hormones. 

Research suggests that vegans tend to eat below the recommended amount of iodine and have low iodine status1,2.  

 

The National Institutes of Health's Recommended Dietary Allowances (RDAs) for Iodine:  
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     *Adequate Intake (AI) 

 

Iodine deficiency during pregnancy can lead to developmental delay and intellectual disability in children.  
Iodine deficiency can cause hypothyroidism (an underactive thyroid). Excessive iodine intake can lead to hyperthyroidism (an overactive thyroid). Both hyper- and hypo-thyroidism can lead to a goitre (an enlarged thyroid gland which can present as a lump in the front of the neck).  

Aside from certain seaweeds, unfortified animal-free foods seem to contain low amounts of iodine.  
 
I do not recommend the use of seaweeds due to the unreliability of the iodine content in these products:  this variability may result in under- or over-dosing. The amount of iodine found in macroalgae products (including supplements) can be alarmingly high; sufficient to lead to toxicity3,4.  
Seaweed was identified as a source of iodine among vegans with excessive iodine intakes1.

The following are the best sources of iodine found in the UK that I'm aware of:   
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  (Source: https://fdc.nal.usda.gov/ + food labels) 

 
If you are unable to meet the RDA for iodine, you should take a supplement.

If you take medication make sure you run this by your doctor. 

 

 

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References

1. Eveleigh, E.R., Coneyworth, L.J., Avery, A. and Welham, S.J., 2020. Vegans, Vegetarians, and Omnivores: How Does Dietary Choice Influence Iodine Intake? A Systematic Review. Nutrients, 12(6), p.1606.  (https://www.mdpi.com)
2. Sobiecki, J.G., Appleby, P.N., Bradbury, K.E. and Key, T.J., 2016. High compliance with dietary recommendations in a cohort of meat eaters, fish eaters, vegetarians, and vegans: results from the European Prospective Investigation into Cancer and Nutrition–Oxford study. Nutrition Research, 36(5), pp.464-477.       (https://www.sciencedirect.com)
3.  Aakre, I., Solli, D.D., Markhus, M.W., Mæhre, H.K., Dahl, L., Henjum, S., Alexander, J., Korneliussen, P.A., Madsen, L. and Kjellevold, M., 2021. Commercially available kelp and seaweed products–valuable iodine source or risk of excess intake?. Food & Nutrition Research, 65.  
(https://www.ncbi.nlm.nih.gov)
4. Bouga, M. and Combet, E., 2015. Emergence of seaweed and seaweed-containing foods in the UK: focus on labeling, iodine content, toxicity and nutrition. Foods, 4(2), pp.240-253.  (https://www.mdpi.com)