Buried deep in the medical literature of the early 20th century is a fascinating study-in rabbits-that, nearly 80 years later, provides some insights into the importance of iodine for heart health.
Back in 1933, Dr. Kenneth Turner of Harvard Medical School conducted a series of experiments using rabbits as a model of atherosclerosis, trying to explore the relationship between thyroid health and heart disease.
In these extensive studies, all rabbits were fed a terrible diet rich in unhealthy fats that caused flagrant atherosclerosis in the aorta (the large artery of the chest and abdomen) within months. (All had intact thyroid function.)
21 rabbits - diet only
This group showed flagrant atherosclerosis, as expected. Blood total cholesterol levels averaged 520 mg/dl.
17 rabbits - given the thyroid hormone, T4 (similar to present-day levothyroxine and Synthroid®)
This group showed approximately half as much atherosclerosis as the diet only group. Total cholesterol averaged 399 mg/dl.
30 rabbits - given desiccated thyroid (roughly equivalent to present-day Armour® thyroid tablets) and iodine
This group showed atherosclerosis in only two rabbits. Total cholesterol averaged 178 mg/dl (66% less than control rabbits).
12 rabbits - given iodine only (potassium iodide)
In this group, one rabbit showed atherosclerosis. Total cholesterol averaged 183 mg/dl.
Okay. That's rabbits. Maybe or maybe not relevant to us humans. Any such studies in humans?
Unfortunately, no. What we have instead are studies that suggest that low thyroid hormone levels, or hypothyroidism, which may develop from iodine deficiency, represent a risk factor for heart disease. Iodine deficiency is suggested by increased TSH and reduced free T4 (one of the actual thyroid hormones) on standard thyroid blood tests. (Unfortunately, it is difficult to tease apart iodine deficiency from hypothyroidism without iodine deficiency based just on blood work.)
Some of the studies of low thyroid and its relationship to heart disease:
1971: 247 participants, all with coronary disease, more than 90% with normal thyroid measures. Desiccated thyroid (not iodine) was administered to all, regardless of starting thyroid status.
In addition to improved well-being and greater capacity for exercise, total cholesterol dropped by 22%, regardless of starting thyroid status. Compared to a matched population, there was 58% reduced 5-year mortality rate. (Wren JC 1971).
2000: Rotterdam Heart Study, 1149 females. Those with a thyroid stimulating hormone level of 4.0 or greater (meaning low thyroid activity but within the conventional "normal" range) experienced 230% greater heart attack risk. (No thyroid hormone was administered in this study.)
2008: The recent HUNT Study is the most revealing. In several analyses, the enormous (30,000 participants) HUNT data showed that: