Increased blood levels of cholesterol that are seen in hypothyroidism are most likely due to
**Question:** Increased blood levels of cholesterol that are seen in hypothyroidism are most likely due to
A. Decreased lipoprotein lipase activity
B. Increased hepatic cholesterol synthesis
C. Increased lipoprotein lipase activity
D. Decreased lipoprotein lipase and hepatic cholesterol synthesis
**Core Concept:**
Hypothyroidism is a condition characterized by underactive thyroid gland, leading to decreased synthesis of thyroxine (T4) and triiodothyronine (T3). These hormones play a crucial role in regulating various body functions, including lipid and lipid protein synthesis. In hypothyroidism, the following changes occur:
1. Decreased lipoprotein lipase activity: Lipoprotein lipase (LPL) plays a vital role in the breakdown of triglycerides in chylomicrons and very low-density lipoprotein (VLDL) particles into their constituent fatty acids and glycerol, which are then utilized by peripheral tissues. Decreased LPL activity leads to impaired triglyceride clearance, which in turn results in elevated triglyceride levels. However, this does not explain the increased cholesterol levels seen in hypothyroidism.
2. Increased hepatic cholesterol synthesis: In hypothyroidism, there is decreased thyroid hormone action on the liver, leading to increased cholesterol synthesis. The liver synthesizes cholesterol that is then incorporated into low-density lipoprotein (LDL) particles, which are released into the bloodstream. These increased LDL particles contribute to the elevated cholesterol levels observed in hypothyroidism.
3. Decreased LDL clearance: Hypothyroidism may also lead to decreased LDL clearance due to decreased LDL receptor expression in the liver. This results in prolonged circulation of LDL particles and contributes to elevated cholesterol levels.
4. Decreased lipoprotein lipase activity does not directly explain the increased cholesterol levels seen in hypothyroidism.
**Why the Correct Answer is B:**
Among the given options, only option B ("Increased hepatic cholesterol synthesis") directly explains the increased cholesterol levels seen in hypothyroidism. The other options do not accurately describe the mechanism leading to elevated cholesterol in hypothyroidism.
**Why Each Wrong Option is Incorrect:**
A (Decreased lipoprotein lipase activity): As mentioned earlier, decreased lipoprotein lipase activity is associated with elevated triglycerides, not cholesterol levels.
C (Increased LDL clearance): Although decreased LDL clearance may contribute to elevated cholesterol, it is not the primary mechanism responsible for increased cholesterol levels in hypothyroidism.
D (Decreased LDL receptor expression): Decreased LDL receptor expression can lead to decreased LDL clearance, but it does not solely explain the increased cholesterol levels seen in hypothyroidism.
**Clinical Pearl:**
Hypothyroidism is associated with multiple changes in lipid and lipoprotein profiles, including increased triglycerides and total cholesterol, along with decreased high-density lipoprotein (HDL) cholesterol levels. The fundamental mechanism involves the reduced thyroid hormone action on the liver, leading to altered lipid and lipoprotein synthesis and clearance. In summary, option B ("Increased hepatic cholesterol synthesis") is the correct answer as it directly