Which of these disorders is when the thyroid is under-active, which leads to decreased metabolism?

Hypothyroidism!

Explanation:

-- Is when your thyroid does not produce enough of the thyroid hormone. This makes your metabolism slow down, affecting your entire body.

Hypothyroidism – underactive thyroid

Explanation:

Hypothyroidism is a condition where your thyroid is underactive and releases too little T4 and T3 hormones into your bloodstream. This causes your metabolism to slow down too much and reduces the thyroid's ability to make hormones.

B. The conclusion paragraph

Explanation:

This paragraph seems to summarize key points and ends with a "call to action," so it's most likely the conclusion paragraph.