Subclinical hypothyroidism is the condition in which T4 levels remain in the normal range (that is, frank hypothyroidism is not present), but TSH levels are elevated: the high TSH levels are required in order to maintain that normal T4 level. So subclinical hypothyroidism implies that the thyroid gland itself is not functioning entirely normally. It's only by "whipping" the thyroid (with high TSH levels) that adequate T4 levels can be maintained.
There is quite a bit of controversy today regarding the true significance of subclinical hypothyroidism. Because the T4 levels are in fact normal in this condition, in theory subclinical hypothyroidism should not be much of a problem. But evidence suggests that subclinical hypothyroidism does produce clinical problems at least sometimes — and so, at least in some cases, it should be treated.
Perhaps the biggest concern with subclinical hypothyroidism is that it may affect the heart.
How is Subclinical Hypothyroidism Diagnosed?This condition is diagnosed with blood tests, specifically, by measuring T4 levels and TSH levels. Subclinical hypothyroidism is present if the T4 levels are in the normal range (0.8 to 1.8 ng/L), and the TSH levels are above the normal range (0.3 to 3.04 mU/L). However, many experts consider subclinical hypothyroidism to be "significant" only when the TSH levels are substantially elevated: above 10 mU/L.
What Causes Subclinical Hypothyroidism?It is believed that subclinical hypothyroidism is merely a mild form, or an early form, of the disorders that commonly produce frank hypothyroidism — most typically autoimmune thyroiditis (Hashimoto's thyroiditis). Indeed, over time, as many as half the people with subclinical hypothyroidism will go on to develop frank hypothyroidism, with low T4 levels and all the symptoms that go along with it. So, one reason some doctors treat subclinical hypothyroidism is to prevent the eventual development of the much more serious condition of true hypothyroidism.
Does Subclinical Hypothyroidism Produce Symptoms?While this condition apparently doesn't produce symptoms in most people, some will admit to mild symptoms suggestive of hypothyroidism, such as constipation, fatigue or unexplained weight gain. It's also been suggested that people with subclinical hypothyroidism may have a higher incidence of significant anxiety, depression, or cognitive disorders.
Does Subclinical Hypothyroidism Affect the Heart?Aside from the risk of eventually developing frank hypothyroidism, the chief worry with subclinical hypothyroidism is that it is associated with an increased incidence of cardiovascular disease.
Several studies have now shown an association between high TSH levels (greater than 10 mU/L) and the development of coronary artery disease (CAD). In a pooled analysis of several clinical studies which included more than 25,000 patients, subclinical hypothyroidism was associated with an increased incidence of heart attack, symptomatic CAD, and cardiac death. Another pooled analysis found a significantly increased risk of heart failure in patients with subclinical hypothyroidism. And several studies have associated subclinical hypothyroidism with increased cholesterol levels.
While an association does not prove cause-and-effect, it is notable that overt hypothyroidism certainly does produce significant cardiac disease. This fact gives credence to the idea that subclinical hypothyroidism may also negatively impact the heart. The increase in cardiac risk seen with subclinical hypothyroidism is indeed the most worrisome feature of this condition.
Should Subclinical Hypothyroidism Be Treated?Subclinical hypothyroidism can be treated by giving thyroid hormone replacement therapy. Treatment is guided by carefully monitoring TSH blood levels; sufficient thyroid hormone is given to reduce TSH levels back into the normal range.
Similarly, evidence that treating subclinical hypothyroidism reduces the risk of heart disease is also limited. In a study conducted in Great Britain, among younger patients (less than 70 years of age) with subclinical hypothyroidism, the risk of subsequent heart disease was significantly lower in patients who received thyroid hormone therapy. No treatment benefit was seen in older patients with subclinical hypothyroidism.
Further, treating subclinical hypothyroidism significantly improves several cardiovascular risk factors, including cholesterol levels, CRP levels, and vascular function.
The Bottom LineMost experts recommend treating subclinical hypothyroidism when TSH levels are greater than 10 mU/L, whether or not symptoms are present.
Whether subclinical hypothyroidism ought to be treated when TSH levels are less than 10 mU/L remains a point of contention. Many doctors recommend treatment even in this lower range if patients have symptoms suggestive of hypothyroidism, or if they have elevated cholesterol levels or other risk factors for cardiac disease.
Should You Be Tested?In the old days (a decade or two ago), thyroid function testing was a routine part of a medical evaluation. But in the interest of cost savings, it is often no longer routine.
It's reasonable to ask your doctor to get thyroid blood tests if you have symptoms suggestive of hypothyroidism (see above), if you have elevated cholesterol levels, or if you just want to be sure you're doing everything you can to reduce your cardiac risk.
Rodondi N, den Elzen WP, Bauer DC, et al. Subclinical hypothyroidism and the risk of coronary heart disease and mortality. JAMA 2010; 304:1365.
Gencer B, Collet TH, Virgini V, et al. Subclinical thyroid dysfunction and the risk of heart failure events: an individual participant data analysis from 6 prospective cohorts. Circulation 2012; 126:1040.
Razvi S, Weaver JU, Butler TJ, Pearce SH. Levothyroxine Treatment of Subclinical Hypothyroidism, Fatal and Nonfatal Cardiovascular Events, and Mortality. Arch Intern Med 2012.