From the Guidelines
Soy is generally safe for people with hypothyroidism, but it is crucial to separate soy consumption from thyroid medication by at least 4 hours to avoid potential interference with thyroid hormone absorption. When considering the safety of soy for individuals with hypothyroidism, it's essential to examine the potential effects of soy isoflavones on thyroid function. Although the study by 1 focuses on the cardiovascular benefits of soy protein, it mentions that the FDA stated "the evidence did not support a significant role for soy isoflavones in cholesterol-lowering effects of soy protein," which implies that isoflavones may not have a substantial impact on overall health.
However, for people with hypothyroidism, the primary concern is the potential for soy isoflavones to interfere with thyroid hormone absorption when consumed simultaneously with thyroid medication. To minimize this risk, it is recommended to separate soy consumption from medication intake. Moderate soy consumption, defined as 1-2 servings daily, is unlikely to negatively impact thyroid function in individuals with adequate iodine intake who are on stable thyroid hormone replacement.
Key considerations for soy consumption in hypothyroidism include:
- Separating soy intake from thyroid medication by at least 4 hours
- Being cautious with soy intake if you have subclinical hypothyroidism or iodine deficiency
- Monitoring thyroid symptoms after consuming soy products and discussing any changes with your healthcare provider
- Maintaining adequate iodine intake to minimize the potential inhibitory effect of soy isoflavones on thyroid peroxidase, an enzyme necessary for thyroid hormone production.
From the FDA Drug Label
Do not administer in foods that decrease absorption of levothyroxine sodium tablets, such as soybean-based infant formula [see Drug Interactions Error! Hyperlink reference not valid.] The FDA drug label does not answer the question.
From the Research
Soy Consumption and Hypothyroidism
- The effects of soy intake on hypothyroidism management have been studied, with some research indicating that soy products can interfere with thyroid hormone absorption, potentially worsening hypothyroidism in individuals receiving levothyroxine replacement 2.
- A review of 14 trials found that soy foods or isoflavones had little to no effect on thyroid function in euthyroid, iodine-replete individuals, but may increase the dose of thyroid hormone required by hypothyroid patients 3.
- However, hypothyroid adults do not need to avoid soy foods entirely, but it is essential for soy consumers to ensure adequate iodine intake to minimize the risk of developing clinical hypothyroidism 3.
Case Studies and Clinical Observations
- Two patients with congenital hypothyroidism were observed to have continued hypothyroidism while consuming soy products, despite receiving recommended doses of levothyroxine, highlighting the potential for soy to interfere with thyroid hormone absorption 2.
- In one case, stopping soy formula and decreasing levothyroxine dose led to resolution of hypothyroidism symptoms, while in another case, switching from soy milk to cow milk and adjusting levothyroxine dose helped normalize thyroid function 2.
Thyroid Hormone Replacement Therapy
- The use of liothyronine (T3) in hypothyroidism treatment is still debated, with some studies suggesting that combined levothyroxine and liothyronine therapy may be beneficial for patients with persistent symptoms, but others finding no significant advantages over levothyroxine alone 4, 5, 6.
- A randomized trial comparing L-thyroxine plus liothyronine with L-thyroxine alone found no objective advantage of combination treatment, although some patients preferred the combination therapy 6.