Hypothyroidism - Underactive Thyroid

What is the Thyroid Gland?

The thyroid gland is found in the neck and sits in front of the trachea (also known as the windpipe). The thyroid gland produces hormones and releases them into the bloodstream. The hormones produced in the thyroid gland are used to control and regulate the metabolism in the body. Metabolism refers to how quickly or slowly your body uses energy and how quickly or slowly reactions in your body take place. The thyroid produces 2 hormones that affect metabolism:

• Thyroxine also known as T4
• Triiodothyronine also known as T3

Both of these hormones increase the rate of metabolism in the body and so reactions and processes in the body occur more quickly. Another hormone called thyroid stimulating hormone (or TSH) controls how much or how little of the thyroid hormones are produced in the thyroid. TSH is produced in a different gland in the brain, known as the pituitary gland.

What is Hypothyroidism?

Hypothyroidism is a condition where the thyroid gland does not produce enough hormones. There are lots of different reasons why this can happen. Insufficient levels of thyroid hormones can affect lots of different parts of the body. Hypothyroidism can affect people of any age and gender. It is, however, approximately 20 times more common in women than men, and is also more common in older people.

What Causes Hypothyroidism?

There are lots of possible causes for insufficient production of thyroid hormones:

• Autoimmune Thyroiditis - This is the most likely cause of hypothyroidism in someone from the United Kingdom. The body’s own immune system, which usually helps fight infection, instead attacks the cells of the thyroid. When the cells become damaged they can no longer produce sufficient quantities of thyroid hormones. This process happens gradually and it is not known what triggers the body’s cells to start attacking the thyroid. In some cases, the thyroid gland swells up (this is known as a goitre). If someone has autoimmune thyroiditis and goitre, this is a particular type of autoimmune thyroiditis known as “Hashimoto’s Disease”.

• Non-Autoimmune Thyroiditis - Thyroiditis refers to any inflammation of the thyroid gland. The most common cause is autoimmune problems, as described above; however, there are also other causes. Viral infection and pregnancy can both affect the thyroid gland. Up to 8% of pregnant women will develop hypothyroidism after giving birth. Often the thyroid becomes inflamed and first produces too much thyroid hormone leading to hyperthyroidism. After this a hypothyroid phase develops where the thyroid no longer produces enough thyroid hormone. This usually lasts up to six months but the inflammation can become chronic, or the thyroid can become permanently damaged. In these cases the hypothyroid phase becomes permanent.

• Previous Treatment for Hyperthyroidism - Hyperthyroidism is a condition where the thyroid produces too much thyroid hormone. Some of the treatments for this condition include radioactive iodine therapy, thyroid surgery, and certain medications. It is possible that these treatments permanently affect thyroid hormone production leading to hypothyroidism.

• Iodine Deficiency - Although not common in the United Kingdom, this is the most common cause of hypothyroidism worldwide. Iodine is used by the body in the production of thyroid hormones. If you do not take in enough iodine as part of your diet your body is unable to produce thyroid hormones and hypothyroidism develops. In the United Kingdom iodine is added to salt when it is manufactured, so iodine deficiency is now very uncommon.

• Certain Medications - Some prescribed medications can stop the thyroid gland from functioning properly. These include lithium, a treatment for bipolar disorder, amiodarone, a treatment for certain heart conditions, carbimazole and propylthiouracil, treatments for hyperthyroidism. Certain cough syrups and kelp-based dietary supplements can contain high levels of iodine. If these products are consumed excessively, the high iodine intake can suppress thyroid hormone production leading to hypothyroidism.

• Congenital (Inherited) Hypothyroidism - Some children are born with a problem with their thyroid. The thyroid doesn’t develop properly, or at all, and they therefore do not produce enough, if any, thyroid hormones. In the United Kingdom babies are screened for this shortly after birth, as part of the heel-prick test.

• Problems with Other Glands - The pituitary gland releases thyroid stimulating hormone (TSH) which stimulates the thyroid to produce thyroid hormones. If there is a problem with the pituitary gland then it might stop producing TSH. This means that even a normal thyroid gland will not be stimulated to produce thyroid hormones leading to a condition known as “secondary hypothyroidism”. The pituitary gland receives signals from another area of the brain called the hypothalamus. The hypothalamus produces thyrotropin-releasing hormone (TRH). TRH acts on the pituitary and stimulates it to produce TSH. If there is a problem with the hypothalamus it will not stimulate the pituitary gland to produce TSH. This will then cause the thyroid to stop producing thyroid hormones. If the problem is in the hypothalamus, this is known as “tertiary hypothyroidism”.

What are the Risk Factors for Hypothyroidism?

Hypothyroidism is more common in women and those over the age of 50. Hypothyroidism due to autoimmune thyroiditis is more likely in people who:

• have close family members who have had autoimmune thyroiditis
• have a goitre
• have a past history of a type of hyperthyroidism known as Grave’s Disease.
• have Down’s Syndrome
• have Turner’s syndrome
• have other autoimmune conditions, or close family members with other autoimmune conditions.

What are the Symptoms of Hypothyroidism?

Hypothyroidism slows down the metabolism in the body, usually quite gradually. The symptoms can therefore affect many parts of the body and often develop over time. Typical symptoms include:

• Dry, thinning hair
• Depression
• Poor memory
• Tiredness and lethargy
• Swollen face, hands and feet
• Muscle pain
• Joint pain
• Dry skin
• Sensitivity to cold
• Brittle, thin fingernails
• Constipation
• Weight gain
• Slower movements and speech

Less common symptoms include:

• Thickening of facial features
• Thinning eyebrows
• Reduced sense of taste and smell
• Hoarseness or deepening of the voice
• Slow heart rate
• Heavier periods
• Fertility problems
• Increased risk of miscarriage
• Decreased libido

How is Hypothyroidism Diagnosed?

If symptoms suggest that the problem might be hypothyroidism, it is possible to do a blood test to confirm the diagnosis. The blood test can measure the levels of T3 and T4 and also TSH. In early hypothyroidism the thyroid hormones may be at normal levels, however TSH may be high because the thyroid needs extra stimulation to produce thyroid hormones. If the levels of thyroid hormones are low and TSH is high this means that the thyroid gland is not producing enough thyroid hormones even with extra stimulation from the pituitary gland. These findings confirm a diagnosis of hypothyroidism.

If the level of thyroid hormones is low and the level of TSH is low this means that the body is not producing enough TSH, and therefore the thyroid gland is not being stimulated to produce thyroid hormones. This suggests either secondary or tertiary hypothyroidism and will require further testing. Another blood test will be done to test TRH levels to identify if the problem is with the pituitary gland or the hypothalamus.

What is the Treatment for Hypothyroidism?

Hypothyroidism is treated with a drug known as levothyroxine. This is a life-long treatment which involves taking a tablet on a daily basis. The tablet contains thyroxine (T4) and acts on the body in the same way as T4 produced by the thyroid gland. This means that even though the thyroid gland isn’t producing thyroid hormones, the tablet increases the levels of thyroxine in the body to normal levels. It takes some time to work out what dose of tablet a person needs to reach their normal levels of thyroid hormone. Usually patients are started on a small dose of levothyroxine, and this is increased gradually until the level of thyroxine in the blood is equal to the level of thyroxine in the blood of someone without hypothyroidism.
Although levothyroxine only contains T4, T4 is converted into T3 in the body. Therefore people with hypothyroidism only need to take levothyroxine to reach adequate levels of both T3 and T4.

Once the correct dose has been established patients usually begin to feel much better, however it is important not to stop taking the treatment.

What are the Side Effects of Hypothyroidism Treatment?

There are not many side effects of levothyroxine treatment because it simply replaces a hormone normally found in the body. If, however, too much levothyroxine is taken, this can lead to symptoms of increased metabolism such as sweating, palpitations, diarrhoea, rapid weight loss and increased risk of osteoporosis. To ensure the levels of thyroxine do not get high and cause increased metabolism, patients will have regular blood tests to monitor thyroid hormone levels when the dose of the treatment is being decided. Once the treatment dose is found and the levels of thyroid hormone are stable and within normal limits, patients are tested, usually yearly, to monitor their metabolism rate. If you develop any symptoms of increased metabolism it is important to tell your doctor immediately.

What are the Complications of Hypothyroidism?

Once hypothyroidism is diagnosed and treated symptoms will resolve and it is possible to live a full and healthy life.

If hypothyroidism is left untreated then the symptoms will gradually worsen. This can lead to infertility, high cholesterol, heart disease, and increased risk of miscarriage or having a baby with birth defects. In extreme cases it is possible to develop a life-threatening condition such as heart failure or severe depression. When thyroid hormones reach extremely low levels a condition known as myxedema coma can develop. This is a medical emergency that is fatal if untreated, and usually requires intravenous injection of thyroid hormones.

Are there any Alternative Diagnoses?

Hypothyroidism affects people in different ways and can cause a wide range of symptoms. It can therefore be quite difficult to diagnose. If you think you may have hypothyroidism it is important to discuss this with your own doctor. If blood tests show low levels of thyroid hormones it is very likely that the cause of your symptoms is hypothyroidism. If your symptoms do not improve with treatment it is important to discuss this with your doctor in case there is a different or additional problem.