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What is borderline hypothyroidism?

Borderline hypothyroidism, also known as subclinical hypothyroidism, is a mild form of hypothyroidism where thyroid hormone levels are slightly abnormal but not yet low enough to cause obvious symptoms. The main sign of borderline hypothyroidism is a mildly elevated TSH level, with normal levels of T3 and T4 thyroid hormones.

What causes borderline hypothyroidism?

The most common cause of borderline hypothyroidism is Hashimoto’s thyroiditis, an autoimmune disease where the immune system attacks and damages the thyroid gland. This causes the thyroid to become underactive and unable to produce enough thyroid hormone. Other causes include:

  • Thyroid surgery
  • Radioactive iodine therapy for hyperthyroidism
  • External beam radiation used to treat cancers of the head and neck
  • Certain medications like lithium, interferon-alpha, and interleukin-2
  • Iodine deficiency
  • Inflammation of the thyroid gland (thyroiditis)
  • Congenital hypothyroidism
  • Pituitary gland disorder

What are the symptoms of borderline hypothyroidism?

Many people with borderline hypothyroidism have no obvious symptoms. When symptoms do occur, they are usually milder versions of the symptoms seen in overt hypothyroidism. Possible symptoms include:

  • Fatigue
  • Weight gain
  • Constipation
  • Dry skin
  • Hair loss
  • Impaired memory and concentration
  • Depression
  • Muscle cramps and joint pain
  • Heavy or irregular menstrual cycles in women

How is borderline hypothyroidism diagnosed?

Borderline hypothyroidism is diagnosed through blood tests that measure levels of thyroid-stimulating hormone (TSH) and thyroid hormones T3 and T4. The key findings are:

  • TSH level slightly elevated above the normal range (5-10 mIU/L)
  • Normal free T4 and total T3 levels

Other Tests may include:

  • Thyroid antibodies like thyroid peroxidase and antithyroglobulin to check for autoimmune thyroiditis
  • Cholesterol levels as elevated cholesterol can be a sign of hypothyroidism
  • Complete blood count to check for anemia and other abnormalities

Who is at risk for borderline hypothyroidism?

You may have a higher risk for borderline hypothyroidism if you:

  • Are over age 60
  • Have a family history of thyroid disease
  • Have been treated for hyperthyroidism in the past
  • Have had thyroid surgery or radioactive iodine therapy
  • Have received radiation to the neck or chest
  • Have other autoimmune disorders like type 1 diabetes or celiac disease
  • Have been pregnant or delivered a baby in the past 6 months
  • Have been exposed to high levels of iodine
  • Take medications that interfere with thyroid function

Can borderline hypothyroidism turn into overt hypothyroidism?

Yes, borderline hypothyroidism can progress to overt hypothyroidism over time if left untreated. Each year, around 5% of people with subclinical hypothyroidism will go on to develop overt hypothyroidism. The risk is higher in those who have:

  • TSH levels above 10 mIU/L
  • Detectable levels of thyroid antibodies
  • Symptoms suggestive of hypothyroidism
  • A goiter or enlarged thyroid
  • A family history of thyroid disease
  • Type 1 diabetes or other autoimmune disorder
  • History of radiation treatment or thyroid surgery
  • Women who have delivered a baby in the past 6 months

In overt hypothyroidism, TSH levels are above 10 mIU/L and T4 levels are below the normal range. Symptoms are also more obvious and severe.

Should borderline hypothyroidism be treated?

Treatment is often recommended for borderline hypothyroidism to prevent progression to overt disease. Treatment is especially important for those at higher risk or those with hypothyroid symptoms. Levothyroxine, a synthetic thyroid hormone, is the typical treatment. The dose is adjusted based on TSH levels, thyroid hormone levels, age, and other health conditions. Treatment goals are to normalize the TSH and relieve symptoms.

Here are some general treatment guidelines:

TSH Level Treatment Recommendation
5-10 mIU/L with no symptoms Monitor TSH annually, consider treatment if TSH rises or symptoms develop
>10 mIU/L with no symptoms Treat with levothyroxine
>10 mIU/L with symptoms Treat with levothyroxine

What are the benefits of treating borderline hypothyroidism?

Treating borderline hypothyroidism can:

  • Prevent progression to overt hypothyroidism
  • Resolve symptoms like fatigue, weight gain, and depression
  • Lower cholesterol and improve heart health
  • Improve pregnancy outcomes in women trying to conceive
  • Possibly improve quality of life and cognitive function

However, research on whether treating subclinical hypothyroidism improves health outcomes has shown mixed results overall. Treatment seems most beneficial for those under age 65 and those with TSH levels above 10 mIU/L.

What are the risks of leaving borderline hypothyroidism untreated?

Potential risks of leaving borderline hypothyroidism untreated include:

  • Progression to overt hypothyroidism and worsening symptoms
  • Increased risk of cardiovascular problems like heart disease
  • Adverse effects on fertility and pregnancy
  • Impaired brain development and cognition in children
  • Possible increased risk of depression
  • Increased risk of goiter and thyroid nodules

However, the absolute risks for most of these complications are small. More research is still needed on the long-term effects of subclinical hypothyroidism.

What is the usual course of borderline hypothyroidism?

With early detection and proper treatment, many people with borderline hypothyroidism can maintain normal thyroid function and avoid complications. But in some cases, borderline hypothyroidism may still progress over time. This is more likely when:

  • The initial TSH levels are higher (over 10 mIU/L)
  • Thyroid antibodies are present
  • The person has Hashimoto’s disease
  • The underlying cause cannot be corrected

Your doctor will monitor your condition over time through regular TSH tests to detect any changes in thyroid function. With ongoing monitoring and treatment adjustments as needed, borderline hypothyroidism can often be well-managed.


Borderline hypothyroidism is a mild thyroid disorder where the TSH level is slightly elevated but thyroid hormone levels are still normal. Symptoms are usually minor if present at all. Borderline hypothyroidism is most often caused by Hashimoto’s disease. Without treatment, it can progress to overt hypothyroidism over time, especially in high risk individuals. Levothyroxine is the typical treatment used to normalize the TSH and prevent complications. With proper treatment and monitoring, patients with borderline hypothyroidism often have good outcomes. However, the condition may still worsen over time in some cases.