Inadvertent Excessive Soy Intake as Part of a Weight Loss Program Leading to Rapid Growth of Multinodular Goiter

Background: Over the past few decades consumption of soy products has gained popularity in the U.S. in part due to reports of potential health and weight loss benefits. However, concerns have emerged regarding soy as a potential endocrine-disrupting chemical (EDC) leading to thyroid dysfunction amongst other health issues. Studies have shown a potential association of high soy intake with risk of hypothyroidism and simple goiter (1), but less is known regarding its impact on multinodular goiter. Clinical Case: A 33-year-old female originally from Bolivia without significant medical history presented to our endocrine clinic with complaints of right-sided neck swelling. The swelling was insidious in onset, had gradually increased in size over the past 3-4 months, and become uncomfortable. She denied dysphagia, cough, or shortness of breath. Prior to the onset of symptoms, she had enrolled in a commercial weight loss program. The diet program consisted of limiting caloric intake to multiple meal replacement bars during the day followed by a light dinner of fish and non-starchy vegetables. Examination revealed a palpable right-sided thyroid mass and enlarged thyroid gland. Lab work showed normal TSH, fT4, and T3 levels, positive anti-thyroid peroxidase antibody (anti-TPO Ab) 588 (<=35.0 IU/ml), and negative thyroid-stimulating immunoglobulins (TSI). Ultrasound of the thyroid confirmed the presence of a 4 cm dominant right thyroid nodule and multinodular goiter. Subsequent fine-needle aspiration of thyroid nodule was consistent with benign nodular goiter. Upon further investigation, it was discovered that the meal replacement bars contained a significant amount of soy protein resulting in an excessive intake of 20-40 grams of soy protein daily. We discussed treatment options, and the patient declined thyroid surgery in favor of surveillance following discontinuation of the weight loss products. The patient was advised to consume a varied diet and given follow-up appointments for monitoring. Conclusion: Inadvertent excessive soy intake via meal replacement bars may have triggered the rapid growth of a multinodular goiter in our patient despite reported adequate dietary iodine intake. Soy products often contain isoflavones that may exert an adverse effect on the thyroid by inhibition of TPO, disruption of iodine metabolism, and/or estrogenic activity. We advise that caution be exercised with high soy protein consumption especially in patients with underlying risk factors for multinodular goiter. References: Messina, M. and Redmond, G., 2006. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid, 16(3), pp.249-258.