What Is Hypothyroidism?

Hypothyroidism is a common clinical condition in which the thyroid gland does not produce a sufficient amount of thyroid hormone.^1,15

The thyroid hormone plays an essential role in the development, growth, metabolism, reproduction and other bodily functions in children and adults.²

Lack of thyroid hormone production can lead to bradycardia (slow heart rate, cold intolerance (abnormal sensitivity to cold temperature or environment), depression, infertility, constipation, tiredness, weight gain and even death.^{3, 4, 5}

The hormones produced by the thyroid gland are triiodothyronine (T3), thyroxine/tetraiodothyronine (T4) and calcitonin (A hormone regulating the calcium level). However, thyroid hormone mostly refers to T3 and T4.^12,14

In-depth, T4 comprises about 80% of thyroid hormones required to stimulate  T3, which is the more active hormone.^5,9

The most common cause of all thyroid disorders is environmental iodine deficiency globally (poor intake of iodine can lead to an enlarged thyroid in the neck). Nevertheless, Hashimoto’s disease (chronic autoimmune thyroiditis) is the most common cause in countries with iodine sufficiency.²¹

Overview

The thyroid gland is a small but vital gland in the endocrine system. The endocrine system refers to the glands and organs that produce the hormones that control many necessary body functions involving the development, growth, metabolism, stress response and reproduction.

This system includes the hypothalamus, pineal gland, pituitary gland, thyroid gland, parathyroid glands, thymus, adrenal glands, pancreas, testes, ovaries and placenta (during pregnancy).^6,7,8

The thyroid is located just under your larynx (also known as Adam's apple or near the trachea) and has two halves structured (called lobes). A  narrow piece of thyroid tissue is maintained (known as the isthmus).^9,13,14

The thyroid gland absorbs iodine from food and converts it into triiodothyronine (T3) and thyroxine (T4). The hormones are then released into the bloodstream and are transported throughout the body, exerting different regulatory roles.^5,10 The thyroid is the only organ or gland in the body that can absorb iodine.^11,14

Thyroid disorders have several potential causing thyroid enlargement  (thyroid nodules, goitre and thyroid cancer) or thyroid hormone dysfunction (hypothyroidism and hyperthyroidism/thyrotoxicosis).^9,15

Thyrotropin-releasing hormones (TRH) influence the thyroid gland to release thyroid hormones. TRH migrates from the hypothalamus and thyrotropin or thyroid-stimulating hormone TSH) from the anterior pituitary.^2,3,5,9

 Hypothyroidism can be classified as primary (due to thyroid hormone deficiency), secondary (due to TSH deficiency), or tertiary (due to TRH deficiency deficiency).^1,17,18

The incidence of thyroid dysfunction is estimated to be 30–40% of all patients seen in endocrine clinics worldwide.^16,22

 The prevalence of thyroid disorders depends on various factors such as age, sex, race/ethnicity, geographical factors, and iodine intake.^1,19,23

In the UK, the prevalence of hypothyroidism is 2%.^15,20

The prevalence of treated hypothyroidism increased from 2.3% (1.4 million) to 3.5% (2.2 million) between 2005 and 2014 in the UK, and  it will keep on rising to 4.2% (2.9 million) by 2025.¹⁹

What causes hypothyroidism to trigger?

Women are 5 to 10 times more likely to be affected than men.^15,22 and from the age above 60 years.²⁷

R ace also plays a role in getting diagnosed. Hypothyroidism occurs towards Northern European descendants and Asians; it has to do with  Hashimoto's disease (a chronic autoimmune affecting the thyroid glands). 

Other disease and treatment methods associated with this condition:

• Thyroiditis
• Pre-existing autoimmune disease 
• Pregnant or postpartum, 
• Dietary iodine insufficiency
• Radioactive iodine 
• Previous  thyroid surgery and medications such as lithium used to treat depression and bipolar disorder, and amiodarone used to treat arrhythmia)^{21,24,25,26,32}

The hypothalamic-pituitary axis can stimulate hormones T3 and T4, a complex feedback loop that helps to maintain the right balance of hormones in our body.^29,30

In the case of T3 and T4, the thyrotropin-releasing hormone (TRH) secretes in the hypothalamus, thyrotropin and anterior pituitary.^2,3,5,9

The TSH level is the most valuable initial test for hypothyroidism.^31,34

When combined with T4, hypothyroidism is a clinical,  mild,  subclinical classification. 

For instance, in a test, TSH is above the range level .T4 levels are below the reference range. However, the range does change as the TSH level rises. T4 concentrations are within the normal range.^32,34

Hypothyroidism can be classified based on the site of the lesion and subsequent deficiency of these hormones. In primary hypothyroidism, the lesion is in the thyroid gland and can develop into thyroid hormone deficiency. In secondary hypothyroidism, the lesion in the pituitary causes the TSH deficiency. In tertiary hypothyroidism,  TRH deficiency is caused in the hypothalamus.^1,17,18

Primary hypothyroidism accounts for over 95% of all cases of underactive thyroid.³¹ 

The common cause is Hashimoto's thyroiditis and Iodine deficiency. The condition can be further caused by iatrogenic causes such as radio-iodine treatment, surgery, and radiotherapy to the neck. Drugs such as amiodarone, iodides, lithium and antithyroid medication. Congenital defects such as absence of thyroid gland. Infiltrative diseases such as amyloidosis, sarcoidosis and haemochromatosis.^{21,31,32,33,34}

Secondary and tertiary hypothyroidism are both collectively referred to as central hypothyroidism and can be caused by lesions in the pituitary (such as cancer, infiltrative diseases, infection and radiotherapy) or in the hypothalamus (such as cancer and trauma).^21,31,32,33

What are the signs and symptoms of hypothyroidism?

The symptoms of hypothyroidism are non-specific and differ between individuals, so a high index of suspicion is required, especially in the elderly.^1,33

Symptoms include; 

• Cold intolerance
• Decreased sweating
• Tiredness
• Weight gain
• Constipation 
• Mental and behavioural changes like depression, poor memory, difficulty concentrating
• Deep  hoarse voice, dry and scaly skin, reduced libido (sex drive)
• Pain
• Numbness  and a tingling sensation in the hand and fingers
• Menstrual irregularities, puffiness (swelling) of the face
• Muscle aches and weakness.^3,21,35

An increase in the severity of symptoms might predict hypothyroidism since a change in seven or more symptoms in the past year increases the likelihood of hypothyroidism.¹

Symptoms and signs are usually more significant in overt (clinical) hypothyroidism.^5,21 

Nonetheless, it is necessary to note that because these symptoms are not specific to hypothyroidism, Diagnosis can only be acceptable from biochemical tests.^32,34

What management and treatment are needed towards hypothyroidism?

Levothyroxine can treat hypothyroidism; the World Health  Organisation validates this treatment as an essential medicine for primary health care.^21,33

Levothyroxine is safe and doesn't usually have any side effects if taken at the prescribed dose.²⁸

Once on treatment, the TSH level is monitored every 4-6 weeks till it is stable, and then it is done every year.^32,38

The goals of treatment are to relief symptoms, prevent long-term complications and maintain serum TSH levels within the reference range.^21,32,34

Treatment is usually more beneficial to those with overt hypothyroidism.³⁴

Levothyroxine should be taken by mouth at the same time every day, preferably in the morning, on an empty stomach, and at least 30 minutes before eating.^28,32,38

 If you forget to take a dose, take it as soon as you remember, if this is within a few hours of your usual time. If you don't remember until later than this, skip the dose and take the next dose at the usual time unless advised otherwise by your doctor.²⁸

FAQs

How is hypothyroidism diagnosed?

A biomedical test is a way to confirm a hypothyroidism diagnosis.. TSH level is the best diagnostic test as increased blood concentration always indicates hypothyroidism.^31,34

 If TSH levels are a steady claim, T4 is then measured to differentiate between subclinical and overt hypothyroidism.³²

 In overt (clinical) hypothyroidism, TSH concentration is above the reference range, and T4 concentrations are below the reference range, and in mild (subclinical) hypothyroidism, TSH concentrations are also above the reference range, but T4 concentrations are within the normal range.^32,34

The reference ranges for TSH and T4 differ with age, sex, and ethnic origin.¹

Additional tests are performed to confirm the underlying cause.^32,37

A Full blood count (FBC) and serum B12 level are tested towards pernicious anaemia is suspected. Glycated haemoglobin (HbA1c) assesses for associated type 1 diabetes mellitus. If autoimmune disease is suspected, thyroid autoimmunity includes thyroid peroxidase (TPOAb) or anti-thyroglobulin antibodies, while neck ultrasound scans for thyroid enlargement.

How can I prevent hypothyroidism?

There's no way of preventing an underactive thyroid if your dietary iodine intake regulates the balance, as most cases are caused either by autoimmune diseases or by damage to the thyroid that occurs during some treatments for an overactive thyroid or thyroid cancer. ⁴

Who is at risk of getting hypothyroidism?

Risk factors are sex (commoner in women), age ( frequent above 60 years), race (commoner amongst Caucasians and Asians), positive family history of Hashimoto's thyroiditis or another autoimmune disease, personal history of an autoimmune disease, pregnant or postpartum, dietary iodine insufficiency, treatment with radioactive iodine, previous thyroid surgery and medications (such as lithium and amiodarone).^21,24,25,26

How common is hypothyroidism?

The incidence of thyroid dysfunction is estimated to be 30–40% of all patients seen in endocrine clinics worldwide.^16,22

In the UK, 15 in every 1,000 women and 1 in 1,000 men have an underactive thyroid, while 1 in 3,500-4,000 babies are born with a congenital hypothyroidism.²⁸

When should I see a doctor?

The symptoms of an underactive thyroid are generally nonspecific, so diagnosis requires laboratory confirmation.^1,21,34

 You should visit your doctor if symptoms occur like tiredness, weight gain, abnormal breathing, depression or any other changes in mental status or behaviour, sensitivity to the cold, dry skin and hair, muscle aches if you notice any neck lumps or changes in your voice or if you are trying to conceive or are pregnant already.²⁸

Your GP may refer you to an endocrinologist (a specialist in hormone disorders) if you are younger than 16, are pregnant, trying to get pregnant or have just given birth, or have another health condition which may complicate your medicine (such as heart disease) or if you are taking a medicine known to cause a reduction in thyroid hormones (such as amiodarone or lithium).³⁶

Summary

Hypothyroidism is a common clinical condition in which the thyroid gland does not produce a  sufficient amount of thyroid hormone. The thyroid hormone plays a  role in the development, growth, metabolism, reproduction and other bodily functions of children and adults. 

The common cause is iodine deficiency globally.  However, Hashimoto's disease ( chronic autoimmune thyroiditis) is the most common cause in countries with iodine sufficiency. 

In the UK, 15 in every 1,000 women and 1 in 1,000 men have an underactive thyroid. While 1 in 3,500-4,000 babies are born with congenital hypothyroidism.

Hypothyroidism is classified as primary, secondary, and tertiary, depending on the individual. Risk factors include female gender, age above 60 years, positive family history of Hashimoto's thyroiditis or another autoimmune disease, personal history of an autoimmune disease, pregnant or postpartum, dietary iodine insufficiency, treatment with radioactive iodine, previous thyroid surgery and medications. Signs and symptoms are nonspecific, so diagnosis depends on biochemical findings. TSH level is the best diagnostic test and is combined with the T4 assay to differentiate between subclinical and overt hypothyroidism. Treatment is lifelong with levothyroxine.