Subclinical thyroid conditions: symptoms, diagnosis, and treatment

Subclinical thyroid conditions, sometimes called mild thyroid failure, often fly under the radar. But they’re fairly common, affecting up to one in ten people [1]. And they become even more common in older age. While symptoms are often absent or mild, a subclinical thyroid condition can develop into a full-blown thyroid condition.

But what exactly is a subclinical thyroid condition, and does it need treatment? This guide sheds light on common questions about the condition, its progression, and long-term effects.

This article includes:

• Overview
• Causes of subclinical hypothyroidism
• Causes of subclinical hyperthyroidism
• Symptoms
• Diagnosis
• Consequences and complications
• Treatment
• When to seek advice
   

What is a subclinical thyroid condition?
 

A subclinical thyroid condition, also called mild thyroid failure, is a condition in which levels of thyroid-stimulating hormone (TSH) are mildly abnormal but thyroid hormones T3 and T4 are still within the normal range. This differs from overt hypo- or hyperthyroidism where levels of T3 and T4 are disrupted.

Subclinical hypothyroidism (raised TSH, normal T4) affects about 4–8% of the population and may affect as many as 20% of women over 60. Subclinical hyperthyroidism (low TSH, normal T4) on the other hand is less common, affecting about 2% of the population [2]. With both conditions, they tend to progress into an overt thyroid condition, but up to half of cases will revert back to normal thyroid function [3].

The table gives more information about what your thyroid blood test results could look like if you have a subclinical thyroid condition.  
 

 

You can read more about subclinical thyroid conditions in our blog: subclinical thyroid conditions: symptoms, diagnosis, and treatment.
 

What causes subclinical hypothyroidism?
 

By far the most common cause of subclinical hypothyroidism is an autoimmune condition called Hashimoto’s thyroiditis, in which thyroid peroxidase (TPO) antibodies are usually present. As this condition develops, T3 and T4 levels eventually begin to fall leading to overt hypothyroidism.

Other causes of subclinical hypothyroidism include:

• Radiotherapy
• Drugs — such as amiodarone, lithium, and immune response modulators.
• Undertreated hypothyroidism
• Iodine deficiency or iodine excess
• Following subacute, painless, or postpartum thyroiditis (usually transient)

The definition of subclinical hypothyroidism doesn’t stipulate that raised TSH must be related to thyroid disease (though often it is). But there are other situations where TSH may rise, and it doesn’t necessarily mean you’re at risk of developing a thyroid condition in the future.

The following situations may also cause a raised TSH:

• Obesity
• Older age — mild TSH elevation (4–7 mU/L) may be a normal physiological adaptation to ageing.
• Assay interference (a falsely elevated result due to the way TSH is measured in the lab) — including interference from biotin supplements.
   

What causes subclinical hyperthyroidism?
 

Much like subclinical hypothyroidism, subclinical hyperthyroidism can be caused by autoimmune thyroid disease, namely Graves’ disease.

Other causes of subclinical hyperthyroidism include:

• Multinodular goitre — an enlarged thyroid gland with lumps or nodules that causes overactivity.
• Thyroiditis — where the thyroid becomes inflamed.
• Thyroid adenoma — a benign tumour of the thyroid gland.
• Excessive levothyroxine replacement

Low TSH may also be seen in the first trimester of pregnancy. Subclinical hypothyroidism in pregnancy is associated with a higher risk of miscarriage, especially if thyroid antibodies are present, but it’s unclear whether treatment with thyroxine reduces this risk [4,5].
 

What are the symptoms of a subclinical thyroid condition?
 

The term subclinical refers to a condition with no obvious signs or symptoms. People with a subclinical thyroid condition may have symptoms, but often the symptoms are mild and non-specific, especially when TSH levels are only slightly outside the normal range.

Symptoms of subclinical hypothyroidism include:

• Low mood
• Fatigue
• Weight gain
• Hair loss
• Constipation
• Cold intolerance

Symptoms of subclinical hyperthyroidism include:

• Rapid heartbeat or palpitations
• Tremors
• Sweating or heat intolerance
• Nervousness, anxiety, or irritability
• Weight loss
• Difficulty concentrating
   

How is a subclinical thyroid condition diagnosed?
 

Since subclinical thyroid conditions often have mild or vague symptoms, you might wonder how it’s picked up or diagnosed. The only way to know for sure is with a blood test, and sometimes it may be found incidentally.

Usually, a doctor will test TSH levels first. If your result comes back abnormal, they will usually follow this with a T4 and/or T3 blood test. You can check for subclinical thyroid conditions at home with our Thyroid Function Blood Test.  

Subclinical thyroid conditions may be classified according to your TSH levels. Different grading systems have been proposed, with one example below:
 

Higher-grade subclinical thyroid conditions are associated with a greater risk of complications and progression to overt thyroid disease.
 

What are the consequences and complications of subclinical thyroid conditions?
 

Both subclinical hypo- and hyperthyroidism may progress to full-blown (overt) thyroid disease over time.

Factors which increase the risk of developing overt thyroid disease [6]:

• TSH level — TSH levels that are significantly low or high (<0.1 or >10 mU/L) are more likely to progress and cause complications.
• Presence of thyroid antibodies — raised levels of thyroid antibodies are usually a sign that there is an underlying autoimmune disease and is associated with overt thyroid disease.
• Underlying cause — some causes of subclinical thyroid conditions do not self-resolve and can therefore progress.

Subclinical thyroid conditions on their own may affect health even before progressing to an overt thyroid condition.

Subclinical hypothyroidism is likely associated with [3]:

• Heart disease — the connection is still being debated, but it’s possible that subclinical hypothyroidism may increase the risk of high blood pressure and raised cholesterol in some individuals. In some studies, subclinical hypothyroidism has been associated with heart failure, especially when TSH is greater than 10 mU/L.

Subclinical hyperthyroidism is likely associated with [3,7]:

• Heart disease — including rhythm abnormalities, like atrial fibrillation, increased heart rate, and reduced exercise tolerance. It’s possible the risk of mortality from heart disease is increased in people with subclinical hyperthyroidism.
• Lower bone mineral density — untreated subclinical hyperthyroidism is associated with a decrease in bone density in postmenopausal women, but it’s not clear if the risk of fracture is increased.
   

How are subclinical thyroid conditions treated?
 

Many cases of subclinical thyroid conditions will self-resolve. Treatment is therefore usually on a case-by-case basis, and can depend on many factors, like your age, levels of TSH, and symptoms. Determining the cause can help determine the best treatment.

In the first instance, your doctor may recommend monitoring your TSH level to assess whether it is a temporary problem, especially if you have no symptoms and your TSH is only mildly abnormal.

Your doctor may offer levothyroxine medication if:

• Your TSH result is greater than 10 mU/L and free T4 is within normal limits (on two separate occasions, three months apart).
• Your TSH result is between 4.5 and 10 mU/L, you’re under 65, and you have symptoms of hypothyroidism.

You may be referred to an endocrinologist if:

• You have a goitre, nodule, or other structural problem with your thyroid gland.
• Your doctor suspects you have an underlying endocrine disease, like Addison’s disease.
• You are planning a pregnancy.
• Your thyroid function results are atypical.
• You take a medication like amiodarone or lithium which may be causing thyroid problems.

You may be offered radioactive iodine treatment, a beta-blocker, or anti-thyroid drugs, such as carbimazole, if:

• You have subclinical hyperthyroidism that is likely due to a multinodular goitre, an adenoma, or Graves’ disease.
• TSH tests confirm that the condition is not transient, you are symptomatic, and are at risk of developing complications.
   

When should you seek advice for a subclinical thyroid condition?
 

If you’ve developed new symptoms of thyroid disease, it’s best to see your doctor, who can examine you and order any necessary tests. If you’ve taken a test with us, our doctors will outline the next steps, whether that’s repeating your test in a few months’ time or advising you book in with your GP.

The good news is that many cases of subclinical thyroid conditions resolve with time. But monitoring your thyroid function with a Thyroid Blood Test can ensure things aren’t getting worse.

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References  

1. Prevalence | Background information | Hypothyroidism | CKS | NICE. [cited 15 May 2023]. Available: https://cks.nice.org.uk/topics/hypothyroidism/background-information/prevalence/
2. Wilson GR, R. Whit Curry J. Subclinical Thyroid Disease. afp. 2005;72: 1517–1524.
3. Cooper DS, Biondi B. Subclinical thyroid disease. The Lancet. 2012;379: 1142–1154. doi:10.1016/S0140-6736(11)60276-6
4. Liu H, Shan Z, Li C, Mao J, Xie X, Wang W, et al. Maternal Subclinical Hypothyroidism, Thyroid Autoimmunity, and the Risk of Miscarriage: A Prospective Cohort Study. Thyroid. 2014;24: 1642–1649. doi:10.1089/thy.2014.0029
5. Maraka S, Ospina NMS, O’Keeffe DT, Espinosa De Ycaza AE, Gionfriddo MR, Erwin PJ, et al. Subclinical Hypothyroidism in Pregnancy: A Systematic Review and Meta-Analysis. Thyroid. 2016;26: 580–590. doi:10.1089/thy.2015.0418
6. Hennessey JV, Espaillat R. Subclinical hypothyroidism: a historical view and shifting prevalence. International Journal of Clinical Practice. 2015;69: 771–782. doi:10.1111/ijcp.12619
7. Santos Palacios S, Pascual-Corrales E, Galofre JC. Management of Subclinical Hyperthyroidism. Int J Endocrinol Metab. 2012;10: 490–496. doi:10.5812/ijem.3447