Medicines Q&A from NHS Specialist Pharmacy Services:
The term kelp can refer to a variety of brown sea weeds. The iodine content in kelp products varies considerably, even between products of the same brand. Hypo- and hyperthyroidism disorders have been attributed to the use of kelp products. UK and American resources caution or advise against the concurrent use of levothyroxine and kelp products. Kelp products should be avoided in patients suffering from thyroid disorders regardless of whether they are taking thyroid supplementation.