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Seaweed and levothyroxine

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Taking Napiers Seagreens Organic Hebridean Kelp alongside Levothyroxine. Read more here.


Case Notes

Taking Napiers Seagreens Organic Hebridean Kelp alongside Levothyroxine

by Monica Wilde
Research Herbalist

This is a summary of reported (empirical) experiences of taking Napiers Seagreens supplement alongside levothyroxine by people with an underactive thyroid gland. This is specifically where there is no autoimmune condition (such as Hashimoto’s disease) nor absence of a thyroid nor damaged thyroid gland.


Over half the population is iodine insufficient. A leading cause of underactive thyroid is a lack of iodine in the diet. Although no official statistics exist as to the number of patients attributable to this cause, it is likely that around 80% of cases of underactive thyroid are caused by the effect of iodine insufficiency over a number of years. The recommended treatment by the Royal College of Physicians is monotherapy with levothyroxine. However, from a naturopathic point of view, treating low levels of T4 hormone without addressing the underlying iodine insufficiency is not logical, as taking levothyroxine does not restore a normal state in all tissues (Celi et al., 2011). 

Adequate dietary iodine intake, either through diet or through iodine supplementation, is crucial to  optimum thyroid baseline health. Levothyroxine drug treatment would be better used once iodine intake is sufficient, if there is still a thyroid problem. Correcting iodine insufficiency may avoid unnecessary medicalisation. 


Napiers Seagreens Organic Hebridean Kelp capsules (Ascophyllum nodosum) have been shown to correct urinary iodine insufficiency, stimulate and rebalance TSH levels, with effects measurable after 3 days in healthy volunteers. This was demonstrated through our research with the University of Glasgow (Combet et al., 2014) in a clinical trial on 50 women that has helped us to understand how this works, and how the seaweed matrix releases chelated iodine into the body in a sustained way over a three day period, avoiding the spikes caused by an intake of potassium iodide.

Napiers has patients who take Napiers Seagreens capsules alongside levothyroxine. In some cases, people have no longer needed levothyroxine but this outcome will vary entirely depending on the degree of underactivity, the length of time you have been iodine insufficient or hypothyroid, and the cause. If you have a damaged or no thyroid, you will always need thyroxine. (These notes are not intended for people with an auto-immune condition such as Hashimoto’s disease who are particularly sensitive to iodine and who are best to consult a medical herbalist.)

  • Taking one capsule (500mg) of Napiers Seagreens a day may mean that you end up requiring a lower dose of levothyroxine. 
  • Our experience is that it is not uncommon for an intake of 1 capsule of Napiers Seagreens per day to require a reduction of 50mcg levothyroxine per day.
  • It is best to start taking Napiers Seagreens a month before your next TSH serum level test. That way if there is any adjustment to be made, or if it does not suit you, you will pick it up early. 
  • If the test shows that your TSH levels have changed, your doctor will adjust your dose of levothyroxine. 
  • Do not take your seaweed capsule at the same time as your levothyroxine. If you take your levothyroxine in the morning, take your Napiers Seagreens capsule in the evening and vice versa.
  • For patients who do well on this, some go on to take 2 capsules a day concurrently with levothyroxine. They introduced 1 capsule, waited a month, did the test, and then introduced the second capsule a month before their next test. We have no experience of patients taking more than 2 capsules a day nor of a reduction of greater than 100mcg levothyroxine.

Ideally, you should discuss your intentions with your GP, or consult a medical herbalist who will monitor your condition while you seek the right balance of adjuncts or alternatives to levothyroxine. The approach of a medical herbalist is to support the entire endocrine system and immune system and they may prescribe additional herbs. They will also be able to check any other medication you may be on and may offer a better outcome than over-the-counter herbs can, as each person and their health is unique. You can find a qualified herbalist in the UK via this link.


Please note that this experience is empirical, based on case study observation, and has not been proven in formal randomised controlled clinical trials. We also only have experience using our Napiers product and not kelp in general, nor kelp from other suppliers, which may have a different nutrient profile (see Appendix).


Celi, F.S., Zemskova, M., Linderman, J.D., Smith, et al. (2011). Metabolic effects of liothyronine therapy in hypothyroidism: a randomized, double-blind, crossover trial of liothyronine versus levothyroxine. J Clin Endocrinol Metab, 96(11), 3466-74. 

Combet, E. (2014). Low-level seaweed supplementation improves iodine status in iodine-insufficient women. Br J Nutr. 2014 Jul 9:1-9. PubMed PMID: 25006699. You can access the full paper here 


Each capsule of 500mg Napiers Seagreens Organic Hebridean Kelp typically contains:

Vitamins:  A (antioxidant) 89mcg, B group (including B12, Thiamine, Riboflavin, Niacin, Pantothenic acid, Pyridoxin, Choline and Cobalamin) 4.45mcg, C (antioxidant) 0.625mg, D (Cholecalciferol) 0.005mcg, E (antioxidant) 0.115mg, H (Biotin) 0.15mcg and vitamin K (Menadione) 5mcg. 

Minerals:  Calcium 7mg, Magnesium 3.2mg, Nitrogen 5.25mg, Phosphorus 0.75mg, Potassium 15.5mg, Sodium 13.5mg and Sulphur 15mg.

Amino acids:  Histidine trace, Isoleucine 1.165mg, Leucine 2.65mg, Lysine 1.39mg, Methionine 0.34mg, Phenylalanine 1.33mg, Threonine 1.4mg, Tryptophan trace, Valine 1.315mg, Alanine 1.95mg, Arginine 1.215mg, Aspartic acid 3.6mg, Cysteine 0.45mg, Glutamic acid 0.70mg, Glycine 1.95mg, Proline 1.5mg, Serine 1.125mg, Tyrosine 0.525mg. 

Trace elements:  Antimony trace, Boron 0.03mg, Cobalt 2.7mcg, Copper trace, Fluorine 0.1mg, Germanium trace, Gold trace, Iodine 350mcg, Iridium trace, Iron 350mcg, Lithium trace, Manganese 0.015mg, Molybdenum 0.325mcg, Platinum trace, Rubidium trace, Selenium 0.075mcg, Silicon 0.5mg, Silver trace, Tellurium trace, Titanium trace, Vanadium 1.15mcg and Zinc 20mcg.

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