These are substances that interfere with iodine utilisation or thyroid hormone production and include thiocyanates found in the cabbage family (e.g. cabbage, kale, cauliflower, broccoli, turnips and Brussels sprouts) and in linseed, cassava, millet and soybean — separate intake of iodine and goitrogens where possible.
The actions of this particular goitrogen are two-fold: ingestion of soy appears to inhibit iodine absorption to some extent (particularly when presented in its thyroxine form in the gut) and also high levels of the isoflavones genistein and daidzein can inhibit T3 and T4 production — separate intake of iodine and goitrogens where possible. Particular attention should be paid to minimising soy consumption in individuals taking thyroid hormone supplementation, as it has been shown that soy consumption can increase dosage requirements.
Selenium is intrinsic to the metabolism and activity of the thyroid hormones, facilitating the conversion of T4 to T3 and is also responsible for the only iodine recycling pathway of the body through the action of the deiodinases on excess or unnecessary thyroid hormones to release the iodine— beneficial interaction.
Contraindications and Precautions
Due to the complex and diverse causes of thyroid conditions, it is advised that iodine supplementation should be avoided unless under the supervision of a medical practitioner.
Up until 2006, the American and Australian recommended daily intake of iodine was 150 µg for pregnant women and 170 µg for lactating women; however, as a reflection of new research, the American and Australian RDI levels for pregnancy have been revised. Care should be taken to avoid ingestion of excessive amounts during pregnancy due to suspected links with increased rates of postpartum thyroiditis and other disorders of thyroid function.