There's no doubt that low thyroid function results in fatigue, weight gain, hair loss, along with rises in LDL cholesterol and other fractions of lipids. It can also result in increasing Lp(a), diabetes, and accelerated heart disease, even heart failure.
But how do we distinguish "normal" thryoid function from "low" thyroid function? This has proven a surprisingly knotty question that has generated a great deal of controversy.
Thyroid stimulating hormone, or TSH, is now the most commonly used index of the adequacy of thyroid gland function, having replaced a number of older measures. TSH is a pituitary gland hormone that goes up when the pituitary senses insufficient thyroid hormone, and a compensatory increase of thyroid hormone is triggered; if the pituitary senses adequate or excessive thyroid hormone, it is triggered to decrease release of TSH. Thus, TSH participates in a so-called "negative feedback loop:" If the thyroid is active, pituitary TSH is suppressed; if thyroid activity is low, pituitary TSH increases.
An active source of debate over the past 10 years has been what a normal TSH level is. In clinical practice, a TSH in the range of 0.4-5.0 mIU/L is considered normal. (Lower TSH is hyperthyroidism, or overactive thyroid; high TSH is hypothyroidism, or underactive thyroid.)
The data from a very fascinating and substantial observation called the HUNT Study, however, is likely to change these commonly-held thyroid "rules."
In this study, over 30,000 Norwegians without known thyroid disease were enrolled. TSH levels and lipid (cholesterol) levels were measured.
In this large and extraordinary observation, increasing TSH levels were associated with increasing levels of LDL cholesterol and triglycerides, and decreasing HDL. At what level of TSH did this relationship start? At TSH levels as low as 1.0!
In other words, there were perturbations in standard lipid measures even with TSH levels ordinarily regarded as "normal," even "perfect."
A subsequent observation from the HUNT Study was even more recently published:
Background Recent studies suggest that relatively low thyroid function within the clinical reference range is positively associated with risk factors for coronary heart disease (CHD), but the association with CHD mortality is not resolved.
Methods In a Norwegian population-based cohort study, we prospectively studied the association between thyrotropin levels and fatal CHD in 17 311 women and 8002 men without known thyroid or cardiovascular disease or diabetes mellitus at baseline.
Results During median follow-up of 8.3 years, 228 women and 182 men died of CHD. Of these, 192 women and 164 men had thyrotropin levels within the clinical reference range of 0.50 to 3.5 mIU/L. Overall, thyrotropin levels within the reference range were positively associated with CHD mortality (P for trend = .01); the trend was statistically significant in women (P for trend = .005) but not in men. Compared with women in the lower part of the reference range (thyrotropin level, 0.50-1.4 mIU/L), the hazard ratios for coronary death were 1.41 (95% confidence interval [CI], 1.02-1.96) and 1.69 (95% CI, 1.14-2.52) for women in the intermediate (thyrotropin level, 1.5-2.4 mIU/L) and higher (thyrotropin level, 2.5-3.5 mIU/L) categories, respectively.
Conclusions Thyrotropin levels within the reference range were positively and linearly associated with CHD mortality in women. The results indicate that relatively low but clinically normal thyroid function may increase the risk of fatal CHD.
In other words, the findings of this substantial observation suggest that the ranges of TSH usually regarded as normal contribute to coronary events, cardiac death, as well as lipid patterns. While several other studies have likewise shown a relationship of higher TSH/lower thyroid function with lipid abnormalities and overt heart disease, no previous study has plumbed the depth of TSH to this low level and to such a large scale.
I believe that these findings are enough cause to begin thinking seriously about monitoring thyroid function more seriously to uncover "borderline" TSH increases in the "normal" range. While higher TSH levels predict cardiovascular events, does thyroid replacement at these levels reduce it? Critics will say it's a big leap, but I think that it is worth at least considering.
Stay tuned for a lengthy Special Report followed by a full booklet on these issues on the www.trackyourplaque.com website.