From Sluggish to Overactive: How Thyroid Hormone Levels Drive LDL, HDL, and Cholesterol Changes

Healthy thyroid function keeps cholesterol balanced and supports cardiovascular well-being.

When most people think about the thyroid, they associate it with metabolism, energy, and weight changes.  But one of the most important — and often overlooked — roles of thyroid hormones is how they influence cholesterol balance in the body.  Both low and high thyroid function can have a significant impact on LDL, HDL, and overall cholesterol levels.

The Thyroid–Cholesterol Connection

The thyroid produces two main hormones: thyroxine (T4) and triiodothyronine (T3).  These hormones regulate how quickly your body uses energy, but they also play a key role in cholesterol metabolism by:

• Controlling the number of LDL receptors on liver cells (which clear LDL cholesterol from the blood).

• Regulating the enzyme that converts cholesterol into bile acids (cholesterol 7α-hydroxylase).

• Influencing the activity of enzymes that process triglycerides and HDL cholesterol.

Low Thyroid Hormone (Hypothyroidism) and Cholesterol

When thyroid function is low:

• LDL cholesterol rises → Because fewer LDL receptors are made in the liver, less cholesterol is cleared from the bloodstream.

• Total cholesterol increases → Both LDL and triglycerides may go up.

• HDL cholesterol may decrease or remain unchanged → The balance shifts unfavourably, raising cardiovascular risk.

• Triglycerides may rise → Due to reduced activity of lipoprotein lipase, an enzyme that helps break them down.

Result: Hypothyroidism is associated with a higher risk of atherosclerosis and heart disease if untreated.  This is why unexplained high cholesterol sometimes leads doctors to check thyroid function.

High Thyroid Hormone (Hyperthyroidism) and Cholesterol

When thyroid function is high:

• LDL cholesterol decreases → Extra thyroid hormone increases LDL receptor activity, clearing LDL more rapidly.

• Total cholesterol falls → Faster cholesterol turnover reduces circulating levels.

• HDL cholesterol may decrease slightly → The protective “good” cholesterol can also drop, though less dramatically than LDL.

• Triglycerides often fall → Due to increased breakdown and clearance.

Result: Hyperthyroidism generally lowers cholesterol levels. While this may sound positive, it comes at the expense of an overactive metabolism, muscle breakdown, weight loss, and heart rhythm problems.

Why Balance Matters

Both low and high thyroid hormone levels disturb the normal cholesterol balance — just in opposite ways.

• Low thyroid hormone (hypothyroidism): pushes cholesterol higher, raising cardiovascular risk.

• High thyroid hormone (hyperthyroidism): lowers cholesterol but can cause other serious health problems.

The key is achieving a healthy thyroid balance, which helps maintain cholesterol at safe levels while also supporting energy, metabolism, and overall well-being.

Takeaway

• The thyroid and cholesterol are closely linked through the body’s metabolic pathways.

• Hypothyroidism → ↑ LDL, ↑ total cholesterol, ↑ triglycerides, normal/↓ HDL.

• Hyperthyroidism → ↓ LDL, ↓ total cholesterol, ↓ triglycerides, slight ↓ HDL.

• If you have abnormal cholesterol levels that don’t seem to match your diet or lifestyle, it may be worth discussing thyroid testing with your Functional Medicine healthcare provider.