**Core Concept**
Type I 5'-monodeiodinase is an enzyme involved in the conversion of thyroxine (T4) to triiodothyronine (T3), the more active form of thyroid hormone. This enzyme plays a crucial role in regulating thyroid hormone levels in the body.

**Why the Correct Answer is Right**
Decreased activity of type I 5'-monodeiodinase leads to reduced conversion of T4 to T3, resulting in decreased levels of T3. This condition is known as T3 hypothyroidism. The decrease in T3 levels can cause various physiologic effects, including decreased metabolic rate, decreased cardiac output, and decreased body temperature. Additionally, decreased T3 levels can lead to decreased expression of genes involved in glucose and lipid metabolism, further contributing to the metabolic slowdown.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased T4 levels. While decreased type I 5'-monodeiodinase activity does lead to increased T4 levels, it is not the correct answer because the question asks for physiologic effects, not hormone levels.
**Option B:** Increased peripheral conversion of T4 to T3. This option is incorrect because decreased type I 5'-monodeiodinase activity leads to decreased conversion of T4 to T3, not increased.
**Option C:** Increased thyroid-stimulating hormone (TSH) levels. While TSH levels may increase in response to decreased T3 levels, this is not a direct effect of decreased type I 5'-monodeiodinase activity.

**Clinical Pearl / High-Yield Fact**
Decreased type I 5'-monodeiodinase activity can lead to a condition known as T3 hypothyroidism, which can be caused by various factors, including certain medications, liver disease, and genetic mutations. It is essential to diagnose and treat T3 hypothyroidism promptly to prevent long-term complications.

**Correct Answer:** D. Decreased peripheral conversion of T4 to T3 leads to decreased levels of T3, causing physiologic effects such as decreased metabolic rate and decreased cardiac output.