Introduction

The thyroid gland performs a vital function: It produces the hormones that regulate the metabolism. Thyroid hormones direct many of the body’s processes.

An underactive thyroid does not produce enough hormones. This condition is also referred to as hypothyroidism. This can lead to various problems: It slows down the metabolism, which may cause fatigue, lethargy, or constipation.

Chronic inflammation of the thyroid gland is the most common cause of an underactive thyroid. It can be treated easily by taking a tablet containing the thyroid hormone once a day. Once the hormone has been replaced the symptoms usually disappear.

Symptoms

Metabolism slows down if the thyroid does not produce enough hormones. That can affect various parts of the body and cause a number of different symptoms. These include the following:

• Exhaustion, weakness and fatigue
• Low pulse
• Sensitivity to cold and reduced sweating
• Shortness of breath when exercising
• Slight to moderate weight gain
• Problems with concentration or memory, slow thoughts or speech
• Constipation
• Hair loss, dry skin and hair
• Yellowish skin
• Low, hoarse voice
• Enlarged tongue
• Thick, “doughy” skin with swelling
• Enlarged thyroid (goiter)
• Poor hearing
• Muscle or joint pain
• Slow reflexes
• Heavy menstrual bleeding or other menstrual cycle problems, low fertility
• Erectile dysfunction
• Lethargy
• Depression or other psychological problems

Many of these symptoms may also have other causes. An underactive thyroid can be mistaken for age-related changes such as the early stages of dementia, especially in older people. This is why a blood test is needed for a reliable diagnosis.

Causes

Thyroid disorders and other factors can prevent the thyroid gland from producing enough hormones. These include the following:

• Hashimoto’s thyroiditis: In this autoimmune disease of the thyroid gland the immune system mistakenly identifies thyroid cells as not being part of the body and attacks them. This causes inflammation in the thyroid gland, but it does not spread to other parts of the body. Hashimoto’s thyroiditis is the most common cause of hypothyroidism.
• Thyroidectomy or radiation therapy: As part of treatment for an overactive thyroid or thyroid cancer, radiation may be used on the thyroid gland or, in a surgical intervention called thyroidectomy, parts or all of it might be removed. An enlarged thyroid (goiter) is also treated by removing parts of it, which results in slowing or stopping the production of thyroid hormones. If radiation therapy was used on the thyroid, hypothyroidism may develop very gradually and might not be noticeable until several years later.
• Severe iodine deficiency: The trace element iodine is needed to make thyroid hormones, but your body can't produce iodine on its own. A diet that contains too little iodine can cause hypothyroidism over the long term, but this is quite rare in industrialized countries nowadays.
• Medication: Some medications slow the production of thyroid hormones. These medications include a number of cancer drugs and lithium, which is also used to treat some mental illnesses. Drugs used to treat an overactive thyroid also act to prevent thyroid hormone production. They might cause hypothyroidism if the dose is too high.
• Central hypothyroidism: Less than 1% of the time hypothyroidism is caused by a disorder of the pituitary gland or the hypothalamus. They both produce hormones that in turn influence production of hormones in the thyroid and are part of the central nervous system.

Outlook

The cause of an underactive thyroid will determine what the outlook is like. Hypothyroidism usually develops gradually in adults because it's most commonly caused by Hashimoto’s thyroiditis. This condition typically ends up slowly destroying the tissue of the thyroid gland.

The remaining tissue can make up for the loss for quite a while by increasing hormone production to keep the body’s levels stable. But if too much of the thyroid tissue has been destroyed, not enough hormones can be produced, which in turn causes the symptoms of hypothyroidism.

The thyroid hormones the body lacks can easily be replaced by taking tablets. This can stabilize the metabolism and make the symptoms disappear. But left untreated, an underactive thyroid can cause a number of different problems. In the rarest of cases it can even lead to a life-threatening coma.

Diagnosis

Your doctor will ask about your symptoms and examine the thyroid by touch. If you show signs of an underactive thyroid, a blood test is done to get a reliable diagnosis.

This test first checks whether your levels of TSH are elevated. TSH means thyroid-stimulating hormone. This hormone is produced in the pituitary gland. It controls the rate at which new thyroid hormones are produced. TSH levels that are just a little too high may be the first sign of the early stages of hypothyroidism: The pituitary gland responds to lower levels of thyroid hormones by increasing TSH production to activate the thyroid.

Experts do not agree on which TSH levels should be considered too high. Many suggest that any value below 4 or 5 milliunits per liter (mU/L) is fine, while others consider levels of TSH of just 2.5 mU/L too high. Sometimes it's is too high for a little while and then comes back down to within the normal range on its own. That's why it's best to repeat the test. If the second test still shows that the TSH level is too high, the amount of “free” thyroxine (FT4) in the blood is measured.

The doctor will diagnose an overactive thyroid if these two things are true:

• You have the typical symptoms of hypothyroidism.
• Your thyroxine level (FT4) is too low.

The normal range used for the thyroxine levels is set by the laboratory analyzing the blood sample. Because the methods for determining the value vary a bit, different labs will use different norms.

Screening

It is especially important for infants and children to have a thyroid that works properly. A lack of thyroid hormones can severely affect a child’s physical and mental development, so levels of thyroid hormones are routinely measured in all newborns. But overall, the likelihood of genetic hypothyroidism is very low: Only about 1 out of every 4,000 newborns is born with a thyroid disorder.

There is no evidence that routinely measuring hormone levels in adults has any benefit. It does have a number of different risks though. Relatively many people have slightly high TSH levels – it is estimated that about 5 to 10% are affected. If TSH is high but the thyroid is still producing enough hormones, the condition is referred to as subclinical (latent) hypothyroidism.

High TSH levels do not automatically mean that someone will develop an underactive thyroid and its related symptoms. On average about 3 to 4 out of 100 people who have high levels of TSH develop noticeable symptoms that need to be treated. The risk of developing symptoms increases if TSH is very high or the blood contains specific antibodies. Women are at a greater risk than men.

Because TSH levels on their own don't tell us much, having them checked regularly could lead to unnecessary treatments and worrying.

And there are more arguments against regularly checking TSH levels without a concrete reason: As of yet, there is no generally accepted threshold for TSH levels being “too high.” Hormone levels also tend to naturally fluctuate. TSH levels that are just a little high usually come back down, but it can sometimes take a while. So TSH levels that are slightly off don't automatically mean that you're ill.

TSH levels also increase somewhat as we age, so taking a different threshold for older people is being considered. Children and teenagers often have slightly higher TSH levels because they're still growing.

Prevention

Getting enough iodine, especially by using iodized salt, is the most important thing you can do to prevent an underactive thyroid. Some dietary supplements also contain iodine. In Germany iodine deficiency is very rarely the cause of hypothyroidism nowadays.

200 micrograms (µg) of iodine per day is recommended for adults – and not more than 500 µg. Children need less than 200 µg of iodine per day, and pregnant or breastfeeding women need a little more.

The Robert Koch Institute studied iodine intake in children and teenagers under the age of 18 in 2007. They found that about 85% of those under 18 got enough or more than enough iodine. There is no comprehensive data on iodine intake in adults.

Getting too much iodine over the long term increases the risk of developing an overactive thyroid. It's virtually impossible though to get too much from food and drink. One exception is dried algae, especially seaweed and kelp. They contain so much iodine that it would be possible to exceed the recommended daily allowance of 500 µg by eating them. Taking dietary supplements containing more than 100 µg iodine on a daily basis can also cause problems.

Treatment

If thyroxine levels are too low, a drug that is similar to the body’s own hormone is used to supplement. The drug is usually referred to as Levothyroxine or L-thyroxine. It keeps hormone levels within the normal range, and symptoms usually clear up completely. Depending on the cause, treatment may be temporary or lifelong.

Thyroxine is taken as a tablet once a day. It is often recommended to take it at least a half an hour before breakfast so the body can absorb the drug better. It can also be taken in the evening before bedtime. This is easier for a lot of people because they won't be eating anything for a while anyway.

At the right dose side effects are virtually nonexistent. The dose will depend on the patient's body weight. Based on how symptoms and thyroid hormone levels develop, the dose can be adjusted after two to three months. It can take that long for the hormone levels in the body to stabilize. If these levels can be kept within the normal range over the long term, they may need to be checked just once a year.

Thyroxine can cause weight loss. People who want to lose weight anyway may feel tempted to take more thyroxine than they need. But this is not a good idea, because too much thyroxine can have side effects like a racing heartbeat, tremors and irritability. It also carries serious long-term risks.

Subclinical hypothyroidism only causes high TSH levels, while thyroid hormones remain at a normal level. Experts don't agree on when it should be treated, but immediate treatment is usually not necessary.

Sources

• Bundesinstitut für Risikobewertung (BfR). Fragen und Antworten zur Jodversorgung und zur Jodmangelvorsorge. February 07, 2012.
• Bundesinstitut für Risikobewertung (BfR). Verwendung von Mineralstoffen in Lebensmitteln. Berlin: BfR Wissenschaft; 2004.
• Pearce SH, Brabant G, Duntas LH, Monzani F, Peeters RP, Razvi S et al. 2013 ETA Guideline: Management of Subclinical Hypothyroidism. Eur Thyroid J 2013; 2(4): 215-228. [PMC free article: PMC3923601] [PubMed: 24783053]
• Rugge JB, Bougatsos C, Chou R. Screening for and Treatment of Thyroid Dysfunction: An Evidence Review for the U.S. Preventive Services Task Force. 10.2014. (U.S. Preventive Services Task Force Evidence Syntheses, formerly Systematic Evidence Reviews).
• Schübel J, Feldkamp J, Bergmann A, Drossard W, Voigt K. Latent Hypothyroidism in Adults. Dtsch Arztebl Int 2017; 114(25): 430-438. [PMC free article: PMC5508068] [PubMed: 28683860]
• Thamm M, Ellert U, Thierfelder W, Liesenkötter KP, Volzke H. Iodine intake in Germany. Results of iodine monitoring in the German Health Interview and Examination Survey for Children and Adolescents (KiGGS). Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz 2007; 50(5-6): 744-749. [PubMed: 17514459]
• UK National Screening Committee (UK NSC). The UK NSC policy on Thyroid disease screening in adults. December 2013.

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