Unawareness of the Effects of Soy Intake on the Management of Congenital Hypothyroidism

Abstract

It has been established that soy products can interfere with thyroid hormone absorption resulting in continued hypothyroidism in individuals receiving recommended levothyroxine replacement. It has also been reported that achievement of euthyroidism in hypothyroid patients using soy products requires increased doses of levothyroxine. We have observed 2 patients with congenital hypothyroidism who continued to manifest clinical hypothyroidism while receiving recommended doses of hormone and ingesting soy products. The first patient was diagnosed by newborn screening (thyroid-stimulating hormone [TSH] =169 µIU/mL) and treated with 50 µg of levothyroxine since 6 days of age while simultaneously starting soy formula. At 3 weeks of age, she was clinically and biochemically hypothyroid (thyroxine = 4.0 µg/dL, TSH = 216 µIU/mL). We stopped her soy formula and decreased her levothyroxine dose. Three weeks later signs of hypothyroidism were resolving, and, by 10 weeks of age, she was clinically and biochemically euthyroid. Another patient was diagnosed by newborn screening, received levothyroxine, and did well. She was lost to us for 2 years. During this interval she began consuming soy milk and became profoundly hypothyroid (free thyroxine <0.4 ng/dL, TSH = 248 µIU/mL), even though the primary care physician had increased her levothyroxine dose to 112 µg/day. She was switched to cow milk, and her thyroid function slowly normalized with decreasing doses of levothyroxine. These 2 patients reinforce the importance of remembering that soy products interfere with levothyroxine absorption and can endanger infants and young children with congenital hypothyroidism who are at risk for developmental and growth delay.

Effects of isoflavones on breast tissue and the thyroid hormone system in humans: a comprehensive safety evaluation

“…A case report published by Fruzza et al ( 2012 ) dealt with the effects of soy-based food in two children suffering from congenital hypothyroidism diagnosed during a new-born screening. Both patients were treated with levothyroxine.…”

“…Upon medical treatment, both patients discontinued consuming the soy-based products. Although in one case readjustment of levothyroxine medication was needed, thyroid parameters in both children normalized (Fruzza et al 2012 ).…”

Dietary recommendations for hypothyroidism and Hashimoto’s disease

“…Po 3 tygodniach od wycofania stosowanej wcześniej mieszanki mlekozastępczej stężęnie TSH spadło do poziomu <50 µIU/mL, a tyroksyny wyniosło >16 mg/dL. 34 Właściwości wolotwórcze może wykazywać również zielona herbata. Zawiera ona w swoim składzie katechiny i flawonoidy, które mogą spowodować osłabienie funkcji tarczycy.…”

Levothyroxine in Psychiatry

“…Soy milk 20 and soy protein 21 can also substantially interfere with T 4 absorption. Administering T 4 in soft gel capsule form has been suggested as a possible way of reducing the effect of coffee on the absorption of T 4 .…”