When should low thyroid be considered and when could it be blamed for contributing to risk for heart disease?
You’d think the answer would be straightforward: Either it’s normal or it isn't.
But the boundary between “normal” and “low” thyroid has been the focus of vigorous debate over the last decade, a question that has vexed doctors and patients alike, often searching for answers to unexplained fatigue, weight gain, swelling, and a host of other troublesome, though non-specific, complaints.
New clinical studies are thrusting this question front-and-center for people interested in correcting abnormal cholesterol patterns and reducing likelihood of heart disease.
The most recent contribution to the discussion is the HUNT Study. What makes this study unique is its unusually large size, drawing thyroid hormone data from 35,000 Norwegians. (The larger the study, the more solid the conclusions.) The HUNT Study is also unique in that it asks the question, “At what level of thyroid function does low thyroid stop contributing to heart attack and distortions of cholesterol?”
Thyroid function is usually assessed by measuring a blood test called TSH (thyroid-stimulating hormone). TSH, as its name suggests, is a (pituitary) hormone that stimulates the thyroid gland’s output of thyroid hormones. If thyroid hormone output is low, TSH increases in an effort to compensate. If thyroid hormone is high, TSH decreases to shut thyroid hormone production back down. Thus, TSH behaves inversely to thyroid hormone: the higher the TSH, the lower the thyroid hormone levels in the body.
Up until a few years ago, a low thyroid was defined as having a TSH of 10 mU/L or greater. Then, based on emerging data, it was reduced to 5 mU/L. Then 4.5. More recently, some authorities and thyroid hormone-taking patients have raised the question of whether 2.5 mU/L is a better cut-off, since some people feel better when thyroid hormone is replaced to this level.
The HUNT Study examined this question along the entire spectrum of TSH levels, from zero on up. At what level did TSH cease to contribute to distortions of cholesterol? A TSH level of 1.0 mU/L! At what level of TSH did a 40% relative increase in heart attack begin to be observed? A TSH level >1.4 mU/L!
In other words, according to the HUNT Study findings, as soon as TSH increases above a cut-off of 1.0 mU/l, it begins to raise LDL cholesterol, reduce HDL (good) cholesterol, and raise triglycerides. There’s also increasing statistical risk for heart attack when TSH exceeds >1.4 mU/L.
Wow. This is big.
If true, it means that significant numbers of people with cholesterol abnormalities and heart disease may have underappreciated thyroid function as an important contributor. On the positive side, it could mean improved health by doing something as simple as having your doctor check thyroid function and discussing whether or not your thyroid levels are ideal in light of the HUNT findings.
What if you are among the many people with higher TSH levels of, say 2.5, 3.0, or 4.5, and you continue to experience unexplained fatigue, hair loss, excessive sleepiness, sluggishness, high LDL cholesterol despite your best efforts? I believe the HUNT data provides ammunition to discuss whether a low thyroid is, indeed, at the root of your problems.
In my clinic and programs, based on these data, we are now aiming for lower and lower TSH levels.
Asvold BO, Vatten LJ, Nilsen TI, Bjoro T. The association between TSH within the reference range and serum lipid concentrations in a population-based study. The HUNT Study. Eur J Endocrinol 2007 Feb;156(2):181-6.
Asvold BO, Vatten LJ, Nilsen TI, Bjoro T. Thyrotropin Levels and Risk of Fatal Coronary Heart Disease: The HUNT Study Arch Intern Med. 2008;168(8):855-860.
Read more from Dr. Davis: