A health professional diagnoses mild, or subclinical, hypothyroidism though a medical history and physical examination. If your health professional suspects you have subclinical hypothyroidism, you will have lab tests to confirm the diagnosis.

Subclinical hypothyroidism is diagnosed when you have:

• No symptoms or mild symptoms of hypothyroidism (such as fatigue, cold intolerance, consistent weight gain, depression, or memory problems).
• A mildly high thyroid-stimulating hormone (TSH) level.
• A normal or only slightly low thyroxine (T4) level.

Some people with subclinical hypothyroidism may test positive for antithyroid antibodies, which indicates they have Hashimoto's thyroiditis, which may cause a gradual loss of thyroid gland function.

Subclinical hypothyroidism should be watched closely. Up to 20% of women over the age of 60 have subclinical hypothyroidism; about 2% to 5% of people with subclinical disease develop hypothyroidism every year.^{1, 2}

Some studies have shown that older adults with subclinical hypothyroidism may be more likely to have heart failure, but more research is needed.

Current research does not provide clear evidence to support treatment of every person with subclinical hypothyroidism, and many doctors disagree whether it should be treated. When making the decision to treat subclinical hypothyroidism, you and your health professional will talk about the benefits of treatment (reduced symptoms) compared to the cost of medicine and monitoring symptoms. Some studies have shown that treatment of subclinical hypothyroidism may lower cholesterol levels, but more research is needed.