Subclinical Hypothyroidism

Subclinical hypothyroidism happens when you have elevated thyroid-stimulating hormone (TSH) levels with normal levels of the hormone thyroxine (T4).

“Subclinical” describes a condition that’s not severe enough to cause definite symptoms. Hypothyroidism happens when your thyroid doesn’t produce enough thyroid hormones. With subclinical hypothyroidism, you don’t technically have hypothyroidism, but it has the potential to develop.

Subclinical hypothyroidism is often temporary but can be long-lasting. It may or may not require treatment.

Symptoms of subclinical hypothyroidism

Most of the time, subclinical hypothyroidism doesn’t cause any symptoms. But you may have symptoms like:

• Fatigue
• Unexplained weight gain
• Constipation
• Depression
• Decreased attention span
• Being unable to tolerate cold temperatures
• Dry, coarse skin and hair
• Diastolic hypertension (high blood pressure)
• Frequent and heavy menstrual bleeding

Subclinical hypothyroidism causes

The typical cause of subclinical hypothyroidism is an underlying issue with your thyroid gland, like Hashimoto’s thyroiditis or another autoimmune disease that causes thyroid inflammation.

Usually, multiple hormones and glands in your endocrine system work together to carefully control the level of TSH in your bloodstream. TSH then stimulates cells in your thyroid to release thyroxine and triiodothyronine into your bloodstream. These two hormones prevent your pituitary gland from producing more TSH if the levels of thyroxine and triiodothyronine are too high.

But in subclinical hypothyroidism, thyroid hormonal output doesn’t increase as it normally should in response to the elevated TSH levels. This leads to elevated TSH levels and normal thyroxine levels.

Risk factors

Some risk factors that contribute to subclinical hypothyroidism include:

• Being female and older than 60
• Personal or family history of thyroid disease
• Positive thyroid antibodies
• Prior or current amiodarone or lithium use
• Iodine deficiency
• Head or neck radiation exposure
• Obesity
• Type 1 diabetes
• Prior treatment for Graves’ disease with antithyroid drugs or ablation

Complications of subclinical hypothyroidism

Most commonly, subclinical hypothyroidism can progress to full-blown hypothyroidism.

Subclinical hypothyroidism may have a connection to an increased risk of cardiovascular diseases like high blood pressure and high cholesterol. But this isn’t a definitive connection.

Does subclinical hypothyroidism affect pregnancy?

Subclinical hypothyroidism affects fewer than 1 in 5 pregnancies in the U.S.

Evidence linking subclinical hypothyroidism to issues during pregnancy is inconsistent and conflicting. Older studies have shown an association between subclinical hypothyroidism in pregnancy and the following conditions:

• Gestational high blood pressure and diabetes
• Preterm labor
• Miscarriage

But more recent studies have not confirmed these findings. Healthcare providers typically only screen for subclinical hypothyroidism during pregnancy if you have risk factors for developing it.

How doctors diagnose subclinical hypothyroidism

Healthcare providers diagnose subclinical hypothyroidism solely on thyroid blood tests.

If your thyroid blood test results show that your TSH levels are elevated (5 to 10 milli-international units per liter or mIU/L) and your thyroxine (T4) levels are in the normal range, it means you have subclinical hypothyroidism.

Providers may categorize subclinical hypothyroidism as grade 1 when TSH levels are between 4.5 and 9.9 mIU/L and as grade 2 if TSH levels are 10 mIU/L or higher.

Does it need to be treated?

It depends on factors like your TSH levels, symptoms and overall health.

On one hand, it makes sense for some people to receive treatment so it doesn’t advance to hypothyroidism. But for others, treatment could potentially cause thyrotoxicosis (too much thyroid hormone in your body), especially in people 65 years or older. Additionally, most people with subclinical hypothyroidism don’t have symptoms.

For most people with subclinical hypothyroidism, providers recommend that they take a “wait and see” approach and not start treatment. However, providers may recommend treatment in the following cases:

• People who have thyrotropin (TSH) levels of 10 mIU/L or higher
• Young and middle-aged people who have symptoms
• Young and middle-aged people who have other risk factors

What is the treatment for subclinical hypothyroidism?

If your healthcare provider recommends treatment for subclinical hypothyroidism, they’ll prescribe a thyroid hormone replacement medication called levothyroxine. It comes in pill form.

Before starting levothyroxine therapy in subclinical hypothyroidism, your provider may order another blood test to check your TSH levels within three months of the first abnormal test result. This is because TSH levels normalize without treatment after three months in many people.

Your provider will likely want you to get routine blood tests to make sure your thyroid levels are in a healthy range while taking medication. If your dosage of levothyroxine is too high, it can cause hyperthyroidism.

Subclinical hypothyroidism treatment for fertility

For women with subclinical hypothyroidism having IVF or ICSI, the American Thyroid Association recommends levothyroxine treatment to achieve a TSH level of 2.5 mIU/L.

When should I see my healthcare provider about subclinical hypothyroidism?

In most cases, healthcare providers take a “wait and see” approach.

If you start to experience symptoms of hypothyroidism, such as fatigue and unexplained weight gain, talk to your healthcare provider. They’ll likely order another thyroid blood test to see if you have overt hypothyroidism.

Can subclinical hypothyroidism be prevented?

In most cases, there’s nothing you can do to prevent subclinical hypothyroidism.

However, if you’re not getting enough iodine in your diet, you can develop subclinical hypothyroidism. This is because your thyroid needs iodine to make thyroid hormone. While this is uncommon in the United States due to the use of table salt, iodine deficiency is the most common cause of hypothyroidism worldwide.

What can I expect if I have subclinical hypothyroidism?

Each case of subclinical hypothyroidism is unique. Many cases resolve on their own within three months.

The risk of subclinical hypothyroidism progressing to overt hypothyroidism is 2% to 6% per year.

You should talk to your healthcare provider about your unique situation to see how they recommend moving forward and how often you should be having blood work.