Endocrine Abstracts

Seaweed consumption results in high levels of iodine intake in Asian populations and influences the presentation of thyroid and perhaps extrathyroidal disorders. The relationship of seawater to seaweed is analogous to that between the bloodstream and the thyroid. However, seaweed has a dramatically more efficient uptake system than the thyroid, concentrating iodide from seawater by a factor of up to 10⁶ compared to 40-50 for the thyroid. The ability of seaweeds to both accumulate and retain iodide prompted a study of iodide transport mechanisms in seaweeds with a view to determining the applicability of these systems to a mammalian cell model. Seaweed discs (1cm² prepared from the kelp Laminaria digitata were incubated in seawater spiked with ¹²⁵I for 4 hours at 4 degC and uptake of radioactivity counted. To measure ¹²⁵I retention discs were then transferred to fresh seawater and after set time points retention of radioactivity in discs was measured. The effects of antithyroid agents on ¹²⁵I uptake by seaweeds was investigated. Dose dependent blockade of uptake from kelp varied from a maximum of 86 percent with perchlorate [1mM], 92 percent with propylthiouracil (PTU) [5 mM] and methyl methimazole (MMI) 72 percent [50 mM]. These findings suggest qualitatively similar mechanisms for iodide transport in both seaweed and the thyroid. Seaweed uptake of iodine appears to require organification as incubation of kelp extracts with 2.5 mM and 5.0 mM PTU resulted in a 25 percent and 48 percent decrease in iodoperoxidase activity respectively. Seaweed also promoted iodide retention as thyroid cell lines incubated with seaweed extracts for 72 hours retained approximately 25 percent more ¹²⁵I than control cells. Understanding the mechanisms of iodide uptake and retention by seaweeds has the potential to assist in enhancing the therapeutic effectiveness of radioactive iodine in thyroid and perhaps extrathyroidal tissues.