Here's why:
* It's not a proper noun: "Iodine" is a common noun, referring to a chemical element. It's not the name of a specific person, place, or thing.
* Outline conventions: Outlines typically use lowercase letters and numbers to organize information. Capitalization is generally reserved for titles and headings.
Example:
Outline:
* I. Introduction
* A. Definition of iodine
* B. Importance of iodine in human health
* II. Sources of Iodine
* A. Dietary sources
* B. Supplements
* III. Iodine Deficiency
* A. Symptoms of iodine deficiency
* B. Health risks associated with iodine deficiency
In summary: Stick to lowercase letters for elements like "iodine" in your paper outline.