What Is Subclinical Hypothyroidism?

Subclinical hypothyroidism ( SCH) is a medical condition where thyroid hormone thyroxine (T4) levels are in the normal range, but thyroid stimulating hormone (TSH) is slightly elevated. This condition is also referred to as mild thyroid failure. It can be temporary or have long-lasting effects. As per studies, around 8% of people assigned female at birth ( AFAB) and 3% of people assigned male at birth (AMAB) have subclinical hypothyroidism in the UK.¹

Hypothyroidism is a medical condition due to underactive thyroid with low levels of T3 and T4 along with elevated levels of TSH. ‘Subclinical’ refers to the phase where there are no definite symptoms as the condition is not so severe. It means that the person is at risk of developing hypothyroidism if proper care is not taken.

The thyroid gland

Let's take a quick look at the thyroid gland anatomy and the different thyroid hormones.

The thyroid gland is an endocrine gland located in the neck region of the body. Endocrine gland refers to a gland which produces hormones, which are chemical messengers. 

The thyroid gland is responsible for producing thyroid hormones and calcitonin. It is essential in maintaining metabolism, growth and maintaining calcium levels in the body.²

The thyroid gland produces three hormones:

• Triiodothyronine (T3): Hormone containing three atoms of iodine. 
• Tetraiodothyronine (thyroxine) (T4): Hormone containing four atoms of iodine. 
• Calcitonin: Helps to maintain calcium levels in the blood.  

A hormone called thyroid stimulating hormone (TSH) or thyrotropin is released by the pituitary gland. This hormone regulates the production of T3 and T4 hormones. 

Symptoms

SCH or hypothyroidism causes the following symptoms:³

• Weight gain
• Constipation
• Fatigue
• Depression
• Inability to tolerate cold
• High blood pressure
• Heavy menstrual bleeding
• Dry and coarse skin
• Irregular periods
• Decreased libido
• Loss of appetite
• Muscle cramps and weakness
• Decreased attention span

Causes and risk factors

Causes of and risk factors for subclinical hypothyroidism or hypothyroidism are:^3,4

• Iodine deficiency: Iodine deficiency in the body is one of the most common causes of SCH. A diet rich in iodine should be followed, unless restricted by medical professionals. 
• Autoimmune thyroiditis: Also known as Hashimoto’s thyroiditis. It is an autoimmune condition where thyroid cells are destroyed by the body’s own immune system. 
• Post-surgical hypothyroidism: Patients who undergo partial or complete thyroidectomy (removal of the thyroid lobe) are at risk of developing hypothyroidism.
• Medication-induced hypothyroidism: Anti-thyroid drugs or radioactive iodine treatment can cause hypothyroidism. 
• Age: Ageing is a risk factor for hypothyroidism, with people above the age of 60 years being at high risk.
• Gender: People AFAB are more prone to developing hypothyroidism compared to people AMAB.  

Diagnosis

SCH can be diagnosed in the following ways:³

• Blood tests: Blood tests are the first step in diagnosis of hypothyroidism. Reports showing elevated levels of TSH with a normal level of T4 is indicative of subclinical hypothyroidism. Several other factors can also increase TSH levels, so these need to be ruled out. 
• Antibodies: A test to check the level of TPO antibodies is helpful in diagnosing hypothyroidism. With autoimmune conditions, there can be an increase in anti-thyroid antibodies which kill the thyroid cells.conditions.
• Ultrasound: Ultrasound is another diagnostic tool for detecting hypothyroidism. 

Management and treatment

SCH can be managed depending on the serum TSH levels. Serum TSH levels in the range 5–10 mIU/L are considered low grade and they might not require any medical intervention. Serum TSH levels of more than 10 mIU/L are considered severe. Treatment options can depend on the age, symptoms and TSH levels.

Levothyroxine therapy: The aim of this therapy is to normalise the TSH levels. This therapy is mostly intended for patients whose serum TSH is more than 10 mIU/L.³

Levothyroxine is a synthetic version of thyroxine (T4). This medication has negligible side-effects and is quite beneficial in treating the condition. The medication dose depends on the patient's condition, age and weight. It is recommended to take this medicine daily, on an empty stomach. Patients should try not to skip their doses and take it in prescribed dosage. Regular thyroid testing may be prescribed depending on the patient's condition. 

Complications

Subclinical hypothyroidism is not a severe condition. Most of the cases are asymptomatic. But, if it is diagnosed at this early stage, proper care must be taken to ensure it does not turn into hypothyroidism. 

Hypothyroidism has some effect on cardiovascular health. It can increase the chances of the following conditions or diseases:^3,5

• Coronary artery disease: Coronary heart disease (CHD) or coronary artery disease or ischaemic heart disease is a condition when coronary arteries are blocked due to fat build-up which leads to reduced blood supply to the heart.  
• Arterial hypertension: It refers to more than 140 mm Hg of systolic pressure or 90 mm Hg of diastolic pressure. Studies have shown a link between increased systolic blood pressure and subclinical hypothyroidism. 
• Hypercoagulation: Subclinical hypothyroidism is linked with elevated C-reactive proteins, raised arterial stiffness and a change in coagulation parameters. When we get injured, our body makes clots to stop the bleeding. This process is called clotting or coagulation. In some cases, blood clots form too easily or don't dissolve leading to blockage of blood flow. This condition is called hypercoagulation or excessive blood clotting. 
• Atrial fibrillation: Atrial fibrillation is a medical condition affecting the heart where a patient suffers from irregular or abnormally fast heart rate. Normal heart rate is between 60-100 beats per minute. It can cause dizziness, shortness of breath or heart palpitations. 
• Heart failure: Heart failure incidents risk is high in cases of subclinical hypothyroidism as ageing-related cardiac changes are accelerated by this condition.
• Functional capacity: Subclinical hypothyroidism leads to decreased muscular strength. This can impact the ability of patients to function properly or endure exercises.
• Cognitive decline: Studies have shown that patients suffering from subclinical hypothyroidism, who are less than 75 years old, are at risk of cognitive impairment. Cognitive impairment refers to a condition where the person's ability to learn new things, remember things or make everyday decisions is altered. It can be mild or severe. 

SCH and pregnancy

During pregnancy, the body goes through lots of hormonal changes. Some pregnant people AFAB get subclinical hypothyroidism. Maternal hypothyroidism can lead to learning difficulties in babies, low birth weight and increased chances of caesarean section. It is essential to monitor serum TSH levels during pregnancy and get thyroid function tests (TFTs) in case of suspicion. 

Summary

The thyroid gland is an endocrine gland which secretes the hormones T3, T4 and calcitonin. TSH regulates the production of T3 and T4. In subclinical hypothyroidism, the TSH levels are elevated while T4 levels are in range. This condition is not severe and may not require medical intervention. But if serum TSH levels are more than 10 mIU/L, levothyroxine therapy is recommended. Treatment and dosage varies according to the age, serum TSH levels and body weight of the affected individual. 

Symptoms of hypothyroidism include heavy and irregular periods, depression, weight gain, reduced attention span, reduced libido, dry and coarse skin, high blood pressure etc. Blood tests and ultrasound are effective ways of diagnosis. It is mostly seen in people AFAB compared to people AMAB. People above the age of 60 years are at high risk of SCH. SCH can lead to complications such as atrial fibrillation, hypercoagulation, coronary artery disease or even heart failure. If SCH is diagnosed, it is better to monitor it regularly and follow the advice of medical professionals to prevent it from turning into hypothyroidism.