Hyperthyroidism (Overactive Thyroid): Signs, Causes, and How It Is Diagnosed

Hyperthyroidism means the thyroid gland, in the front of the neck, is making more thyroid hormone than the body needs [1][14]. Because this hormone helps set how the body uses energy, too much of it speeds many things up, from heart rate to digestion [1][4]. Graves disease, an autoimmune condition, is behind most cases [4][6], and an overactive thyroid is usually treatable [4][8]. It is the opposite problem to an underactive thyroid, which is covered in Hypothyroidism: Symptoms, Causes, and Diagnosis.

What Happens When the Thyroid Works Too Hard

The thyroid makes two main hormones, T4 (thyroxine) and T3 (triiodothyronine), and in hyperthyroidism the levels of both run high [6]. Their effects reach almost every organ, including the heart [1]. Doctors sometimes use the word thyrotoxicosis for too much thyroid hormone in the blood, whether the gland is overproducing or an inflamed gland is releasing hormone it has stored [4][10].

How Common It Is

Figures depend on the population studied. NIDDK, part of the U.S. National Institutes of Health, says about 1 in 100 Americans aged 12 and over have hyperthyroidism [1]. A JAMA patient guide puts the figure at about 2.5 percent of people worldwide [14].

It is much more common in women. The NHS says it is about 10 times more common in women than in men [4]. Age patterns are described differently, mostly because they depend on the cause. The NHS says an overactive thyroid typically starts between the ages of 20 and 40 [4], while NIDDK says hyperthyroidism is more common in people over 60 [1]. Graves disease mostly affects young and middle-aged women [6], while overactive nodules usually appear after 60 [1][6].

Signs and Symptoms to Notice

Few people have every symptom. Signs can come on gradually or suddenly, and they range from mild to severe [5]. JAMA notes that some people have very mild symptoms or none at all [14]. Commonly described signs, grouped by the part of the body they affect, include [1][5][11][14]:

• Heart and breathing: a fast, pounding or irregular heartbeat, and sometimes shortness of breath when lying down.
• Heat and skin: feeling too hot, sweating more than usual, warm skin and, for some people, itching or thinning hair.
• Weight and digestion: weight loss, often despite a bigger appetite, and more frequent bowel movements or diarrhea.
• Mood and sleep: nervousness, anxiety, irritability, mood swings and trouble sleeping.
• Muscles and energy: tiredness, muscle weakness and shaky hands.
• Periods and fertility: irregular periods and difficulty getting pregnant.
• Neck and voice: a swelling at the front of the neck, called a goiter, and sometimes changes in the voice.
• Eyes: irritation, redness or bulging, mainly with Graves disease.

The NHS also mentions persistent thirst and needing to pee more often [5].

Older Adults

In older adults the picture can be different. NIDDK says they may have symptoms such as loss of appetite or pulling away from other people, and hyperthyroidism is sometimes mistaken for depression or dementia. NIDDK suggests asking a doctor about hyperthyroidism if you or a loved one shows these signs [1].

Ruling Out Other Causes

Many of these signs also occur in other conditions, so a diagnosis cannot rest on symptoms alone [1][11]. The NHS says a blood test can often show whether the thyroid is involved, and that listing your symptoms to show a GP may help [5]. If a racing or fluttering heart is your main worry, Heart Palpitations: Causes, Symptoms, and When to Worry explains other common causes.

What Causes an Overactive Thyroid

Graves disease. The NHS says about 4 in every 5 people with an overactive thyroid have Graves disease [4][6]. The immune system makes an antibody that acts like the body's own signal to the thyroid, so the gland keeps making extra hormone [2]. NIDDK says the cause is probably a mix of genes and an outside trigger, such as a virus [2]. Graves disease often runs in families, and smoking can increase the risk [6][12].

Overactive nodules. Lumps in the thyroid are common and usually not cancer, but one or more can start making too much hormone [1]. JAMA describes this as the second most common cause [14].

Thyroiditis. When the thyroid is inflamed, stored hormone can leak into the blood [1]. Thyroiditis can be painful and linked to a virus, can appear after giving birth, or can happen without pain [1][11], and the overactive phase may be followed by an underactive one [1].

Other causes include large amounts of iodine, for example from seaweed or seaweed supplements [1], taking too much thyroid hormone medicine [1][14], and some other medicines [6][11]. Rarely, a non-cancerous growth on the pituitary gland is the cause [1][6]. The NHS also notes that high levels of the pregnancy hormone hCG can sometimes play a part [6].

NIDDK also says it is more likely in people who have some other health problems, such as primary adrenal insufficiency or diabetes, and in people who use nicotine products [1].

Tests Used to Confirm It and Find the Cause

A doctor will usually ask about your symptoms and health history and examine you, including your neck, eyes, blood pressure and any trembling [1][12]. The main blood test measures TSH, the hormone the pituitary gland uses to control the thyroid, together with T4 and T3, which may be reported as free T4 and free T3 [7]. The NHS explains that a low TSH with high T3 or T4, or both, usually means the thyroid is overactive [7].

If the thyroid is overactive, further tests help find the cause [7][10]:

• an antibody blood test, which is usually positive in Graves disease [7][11][14]
• a blood test for inflammation, which can point to thyroiditis [7]
• a thyroid scan, which uses a small amount of a slightly radioactive substance to look for nodules [7], and can show a pattern that fits Graves disease [2]
• an ultrasound in some situations, for example when a scan is not suitable during pregnancy or breastfeeding [2]

NIDDK adds that women who have trouble getting pregnant are often tested for thyroid problems [1].

Graves Eye Disease

Graves disease can affect the tissues around the eyes. The NHS and NIDDK describe dry or gritty eyes, watering, sensitivity to light, redness, swollen or pulled-back eyelids, bulging eyes and blurred or double vision [2][9]. NIDDK notes that eye problems can start before or at the same time as the other symptoms of an overactive thyroid and, rarely, after Graves disease has been treated [2].

Estimates of how often this happens vary. The NHS says it affects around 1 in 4 people with Graves disease [9], and the Endocrine Society's Graves page says about a quarter [12]. The Endocrine Society's eye disease page says about a third of people with Graves disease have some signs [13], and NIDDK says more than 1 in 3 [2]. The Endocrine Society's general hyperthyroidism page, by contrast, calls eye disease relatively rare [11].

Most people have mild symptoms [2], and the NHS says many mild cases improve as the thyroid is treated [9]. In a small number of people the eye disease is severe and can threaten sight [9][13]. Smoking makes eye disease worse, and NIDDK advises stopping and asking for help to quit [2][13]. An eye specialist can assess and treat it [9][13].

Possible Complications if Left Untreated

Some of the most serious problems involve the heart and the bones [11]. An untreated overactive thyroid can cause an irregular heartbeat, such as atrial fibrillation, which can lead to blood clots, stroke and heart failure [1][9]. For more on that heart rhythm problem, see What Causes Atrial Fibrillation? Mechanisms, Triggers, and Risk Factors Explained.

Hyperthyroidism can also thin the bones and lead to osteoporosis [1][9][15]. The International Osteoporosis Foundation advises talking to your doctor about bone health if a risk factor like this applies to you, and asking about an assessment [15]. Secondary Osteoporosis: Causes and When to Suspect It covers other conditions that affect bone strength.

Menstrual and fertility problems are also possible [1]. JAMA adds that untreated hyperthyroidism is linked with pregnancy complications and a higher risk of death [14].

Pregnancy and After Birth

NIDDK says mild hyperthyroidism in pregnancy is usually not a problem, but severe, untreated hyperthyroidism can affect both mother and baby [1]. The NHS says that when it is not well controlled during pregnancy, there is a higher risk of pre-eclampsia, miscarriage, premature birth and low birthweight [9].

It can be harder to spot in pregnancy, because a faster heart rate, feeling hot and tiredness also happen in a normal pregnancy [3]. NIDDK says a fast and irregular heartbeat, shaky hands, or unexplained weight loss or not gaining the expected weight can point to hyperthyroidism [3].

If you have hyperthyroidism and are planning a pregnancy or are pregnant, NIDDK advises working with your doctor to get it under control [1]. If you have ever been treated for Graves disease, tell your doctor, because in rare cases the antibodies can still affect the baby's thyroid and antibody levels may need checking [3]. After birth, postpartum thyroiditis can cause an overactive phase lasting up to 3 months, often with no symptoms or only mild ones [3].

Thyroid Storm

Rarely, an undiagnosed or poorly controlled overactive thyroid can lead to thyroid storm, a sudden and life-threatening flare-up of symptoms [4][9]. The NHS lists a rapid heartbeat, a high temperature, high blood pressure, yellowing of the skin or eyes, severe agitation and confusion, and loss of consciousness [9]. It says triggers include an infection, pregnancy, treatment not being taken as prescribed, and damage to the thyroid, such as a blow to the throat [9].

The NHS describes thyroid storm as a medical emergency and, in the UK, advises calling 999 for an ambulance immediately if you think you or someone in your care has it [9].

How Treatment Is Decided

The NHS says people are usually referred to an endocrinologist, a hormone specialist, who plans treatment [8]. NIDDK says the aims are to bring hormone levels back to normal, prevent long-term problems and relieve symptoms, and that no single treatment works for everyone [1]. The choice depends on the cause and how severe it is [1][11], and JAMA adds pregnancy plans, personal preference and access to an experienced surgeon [14]. In England, NICE says people should be given information about the benefits and risks of each option, including any effect on eye disease, so the decision can be shared [10].

The NHS describes three main types of treatment, each with benefits and drawbacks [4]:

• treatment that lowers the amount of hormone the thyroid makes
• radioactive iodine treatment, which the NHS describes as a type of radiotherapy that destroys thyroid cells
• surgery to remove some or all of the thyroid, used for some people

When thyroiditis is the cause, NICE says supportive care alone is usually enough [10]. Some treatments leave the thyroid underactive afterward, which then needs long-term treatment [1][9]. That outcome is explained in Hypothyroidism: Symptoms, Causes, and Diagnosis.

Questions to Ask Your Care Team

Drawing on Endocrine Society patient guidance [11][12], questions worth asking include:

• What is causing my hyperthyroidism?
• What are the benefits and risks of each option for me?
• How often will I need blood tests?
• Should my heart or bones be checked?
• What should I do if I want to become pregnant?

If you have Graves disease or another autoimmune thyroid condition, NIDDK says large amounts of iodine from foods such as kelp or other seaweed, or from supplements, may make hyperthyroidism worse. It suggests talking with your health care team about foods to limit and about supplements, cough syrups or multivitamins that may contain iodine [1][2].

Conclusion

If several of these signs sound familiar, the NHS suggests seeing a GP, who can arrange a thyroid blood test [4][7]. If you have been treated, now or in the past, the Endocrine Society advises seeing your doctor regularly to make sure your hormone levels stay where they should be [11].

Medical Disclaimer

This article is for general information and does not replace advice from a qualified health professional. If you think you or someone with you needs urgent medical care, get emergency help straight away.