The biggest "reason" I've gotten from Endocrinologists for NOT treating hypothyroidism when the TSH isn't above 5 (in the US) or 10 (in the UK) is Heart Attack and Heart Disease. One Endocrinologist actually had this exchange with me:

Her: I can't give you an increase.
Me: But, I cannot get out of bed or function.
Her: Well, to keep your heart healthy your TSH must be above 3
Me: What good is a healthy heart if I can't use it in real life, and I'm in bed all the time?
Her: You say that now, but when you have a heart attack or cardio-myopathy you'll sue me.
Me: Eye-Roll and thought bubble above my head, "Well, you've probably been sued before because you don't know what you're doing."

The truth is: There's very little evidence that under-treating or not-treating hypothyroidism actually prevents heart attacks or heart disease or cardio-myopathy. In fact, new research shows a strong connection between low T3 levels and heart disease. See for yourselves dearies:

I highly recommend reading this one:
http://thyroid.about.com/cs/symptomsproblems/a/heartrisk.htm


http://www.circ.ahajournals.org/content/108/4/e29.full
Excerpt:
"in assessing the role of thyroid hormones in the prognosis of patients with heart disease, showed that the free triiodothyronine (fT3) level was the highest independent predictor of death during the 1-year follow-up period. The authors stated that clinical and experimental knowledge of the fundamental actions of thyroid hormones, in particular of T3, on both the heart and vessels favors the hypothesis of a direct relationship between low-T3 syndrome and mortality in patients with heart disease.

Overt hypothyroidism, with its accompanying hypercholesterolemia and hypertension, may be associated with cardiovascular disease.2 Moreover, subclinical hypothyroidism is a strong indicator of risk for atherosclerosis and myocardial infarction in elderly women."

http://www.nytimes.com/health/guides/disease/hypothyroidism/print.html
Excerpt: EFFECTS OF HYPOTHYROIDISM AND SUBCLINICAL HYPOTHYROIDISM ON THE HEART
Thyroid hormones, notably triiodothyronine (T3), affect the heart directly and indirectly. They are closely linked with heart rate and heart output. T3 provides particular benefits by relaxing the smooth muscles of blood vessels. This helps keep the blood vessels open so that blood flows smoothly through them.

Hypothyroidism is associated with:

Unhealthy cholesterol levels . Hypothyroidism raises levels of total cholesterol, LDL (the so-called bad cholesterol), triglycerides, and other lipids (fat molecules) associated with heart disease. Treating the thyroid condition with thyroid replacement therapy can significantly reduce these levels.
Mild high blood pressure . Hypothyroidism may slow the heart rate to less than 60 beats per minute, reduce the heart's pumping capacity, and increase the stiffness of blood vessel walls. All of these effects may lead to high blood pressure. Indeed, patients with hypothyroidism have triple the risk of developing hypertension. All patients with chronic hypothyroidism, especially pregnant women, should have their blood pressure checked regularly.
Heart failure . Hypothyroidism can affect the heart muscle’s contraction and increase the risk of heart failure in people with heart disease.
The evidence for subclinical hypothyroidism and heart disease is mixed. Some studies suggest that subclinical hypothyroidism increases the risks for coronary artery disease and heart failure. The only randomized controlled trial dealing with subclinical hypothyroidism and heart disease evaluated only the thickness of atherosclerosis in the blood vessels and not whether patients actually had clinical heart disease. Many doctors believe that treatment of subclinical hypothyroidism will not help prevent or improve heart problems. More research is underway. "

Is your thyroid killing you? http://hypothyroidmom.com/is-your-thyroid-killing-you-heart-disease/