Which of the following is NOT an environmental goitrogen?

• A. Fluoride
• B. Mercury
• C. Perchlorate
• D. Iodide

Answer: (D)

Goitrogens are substances that interfere with thyroid hormone production by disrupting iodine uptake or metabolism in the thyroid. Among environmental goitrogens, perchlorate blocks the sodium-iodide symporter, limiting iodine entry into thyroid cells and promoting goiter in iodine-deficient settings. Fluoride at high exposure has also been implicated in altered thyroid function, which can contribute to goitrogenic effects under certain conditions. Mercury can disrupt thyroid hormone synthesis and metabolism as well. Iodide, however, is essential for making thyroid hormones; the thyroid needs iodide to function properly, so iodide itself does not act as a goitrogen. In fact, adequate iodide supports normal thyroid function, and only in rare, excessive situations might regulatory mechanisms transiently reduce hormone synthesis, but iodide remains not a goitrogen.