Hyperthyroidism, also known as overactive thyroid, occurs when the thyroid gland overproduces thyroid hormones, thus accelerating the body's natural functions. In contrast, hypothyroidism is the result of an underactive thyroid that is not secreting enough thyroid hormones, which leads to the slowing-down of natural functions.

Hypothyroidism is much more common than hyperthyroidism and is usually diagnosed by a blood test measuring the level of TSH (thyroid-stimulating hormone) in the body.

This comparison examines the causes (which can be varied), symptoms (which are often subtle and not obvious), diagnosis, and treatment options for hypothyroidism and hyperthyroidism.

What is the Thyroid?

The thyroid is an endocrine gland found in the neck of vertebrate animals, including humans. It stores, produces, and secretes hormones — triiodothyronine (T3) and thyroxine (T4) — into the bloodstream that regulate numerous functions, including heart rate and blood pressure, body temperature, metabolism, and the growth and development of the brain and nervous system. The brain's pituitary gland regulates the thyroid's hormone secretion with its own hormone known as the thyroid-stimulating hormone (TSH).

Causes of Thyroid Disorders

Other diseases can lead to the development of thyroid problems. In fact, nearly all overactive and underactive thyroid conditions in the U.S. are caused by two specific autoimmune diseases:

• Graves' disease, also known as toxic diffuse goiter, is the most common cause of hyperthyroidism around the world. This disease causes swelling of the thyroid (see goiter) and sometimes of the eyes (see exophthalmos). The thyroid becomes overactive with Graves', releasing too much thyroid hormone into the bloodstream.

• Hashimoto's disease, also known as chronic lymphocytic thyroiditis, is the most common cause of hypothyroidism in the U.S. and much (but not all) of the world. Hashimoto's causes the immune system to mistakenly attack its own, healthy thyroid, slowing its normal functionality until hypothyroidism results.

Other Causes

(Click to enlarge.) Iodine deficiency has become less common since the development of iodized table salt.

Though most cases of hyperthyroidism and hypothyroidism are caused by Graves' and Hashimoto's, thyroid problems can be the result of other events, conditions, or circumstances:

• Thyroiditis — inflammation of the thyroid — can cause either hypothyroidism or hyperthyroidism and usually causes both at different stages. The inflammation itself may be caused by a bacterial or viral infection (subacute thyroiditis), an autoimmune condition (silent thyroiditis), or even childbirth (postpartum thyroiditis). With thyroid inflammation, it is common for a person to develop hyperthyroidism first, followed by hypothyroidism, at which point the thyroid may heal on its own or develop permanent hypothyroidism.

• Diets with too much or too little iodine can seriously affect thyroid hormone production. The thyroid requires the dietary element iodine to properly synthesize T3 and T4 hormones. Too much iodine can cause hypothyroidism; too little, and hyperthyroidism can develop. Thanks to the presence of iodized salt in many (though not all) countries, iodine deficiencies are rare enough that consuming too much iodine may be a more common problem than consuming too little. Due to the importance of iodine in fetal development, however, pregnant women are at slightly greater risk of deficiency (and so, hyperthyroidism) than the general population.

• Medications, such as amiodarone (used for arrhythmia) and lithium (used for bipolar disorder), as well some cough syrups and supplements with seaweed, can cause either thyroid condition.

• It is possible to be born with hypothyroidism (congential hypothyroidism). As such, newborns in the U.S. are screened for this condition.

• Some treatments for hyperthyroidism, such as radioactive iodine treatment and surgical removal of part of the thyroid, can eventually cause hypothyroidism. Full thyroid removal, which is a "last resort" treatment, always results in hypothyroidism.

• Thyroid nodules, lumps in the thyroid, are relatively common and usually benign. However, they can encourage the thyroid to become overactive and release too much thyroid hormone, resulting in hyperthyroidism.

Overactive vs. Underactive Thyroid Symptoms

Both hyperthyroidism and hypothyroidism can cause fatigue, hair loss/thinning, muscle or joint pain, mental anguish (e.g., anxiety and depression, mood swings, or irritability), and many other symptoms that are common to other diseases. Doctors must use other symptoms to evaluate the risk or presence of either disorder and cannot diagnose either without a blood test.

The most common signs and symptoms of hypothyroidism manifest in ways that suggest natural processes of the body are slowed or shutting down:

• Rapid weight gain, despite (sometimes) lack of appetite
• Feeling cold and having cool extremities (hands, feet)
• Slow heart rate
• Decreased sweating
• Dry skin and hair
• Facial swelling or other swelling, such as of the limbs
• Constipation
• In menstruating women, menorrhagia and irregular periods
• Existing diagnosis of an autoimmune disease, such as diabetes mellitus or celiac disease

In contrast, the most common signs and symptoms of hyperthyroidism suggest natural processes are speeding up abnormally:

• Rapid weight loss
• Feeling unnaturally uncomfortable in the heat
• Increased or irregular heart rate
• Excessive sweating
• Diarrhea
• Tremors
• In menstruating women, hypomennorrhea or amenorrhea

Diagnosis

A TSH test is often the first point of diagnosis that medical practitioners use. For this test, blood is drawn and tested for the presence of thyroid-stimulating hormone (TSH). A lab assigns a "normal" range for this hormone — usually between .5 and 4.5 mIU/L. If one's TSH levels fall outside of this normal range, it indicates hypothyroidism (anything above the normal range) or hyperthyroidism (anything below the normal range). It is worth noting that the American Association of Clinical Endocrinologists has recommended a smaller range of .3 to 3.0 mlU/L, which would make a much larger proportion of the U.S. population fall within the hypothyroid diagnosis.[1]

Those who are suspected to have hyperthyroidism may also have their T3 and T4 levels tested, as these levels are higher than normal in the case of hyperthyroidism. Furthermore, while a T3 test is not useful for the diagnosis of hypothyroidism, a lower than normal level of T4 indicates hypothyroidism.

A thyroid-stimulating immunoglobulin (TSI) test is used to check for a particular antibody associated with the Graves' and Hashimoto's diseases. This test helps narrow down the cause of hyperthyroidism and hypothyroidism, whether it is related to these autoimmune disorders or something else.

Two other tests are sometimes employed (and even used together): the thyroid scan and the radioactive iodine uptake test. The simplest thyroid scan, which makes use of ultrasound, is used to look for the presence of thyroid nodules, which can cause hyperthyroidism. More complex scans used for nuclear medicine are sometimes coupled with the radioactive iodine uptake test. For this test, radioactive iodine is injected into the bloodstream and later scanned to see how it has been used by the thyroid.

Treatment of Thyroid Disorders

There is no one cure for either disorder, and what treatment entails can vary from person to person due to the multiple causes of hyperthyroidism and hypothyroidism.

Even so, hypothyroidism is often controlled very well with the use of a synthetic thyroid hormone (e.g., Levothyroxine) or carefully monitored iodine supplementation. Those suffering from hyperthyroidism are typically prescribed an antithyroid medication (e.g., Methimazole) to slow the overactive thyroid and, sometimes, beta blockers (e.g., Propranolol) to alleviate symptoms.

For some, treating a thyroid disorder is a delicate balancing act. Unfortunately, overactive thyroid treatment — particularly more extreme forms of treatment, such as surgery — may eventually lead one to develop an underactive thyroid.

Occurrence

Roughly 1% of the U.S. population has hyperthyroidism. Hypothyroidism is much more common, affecting nearly 5% of the population that is 12 and older. If "normal" ranges for the thyroid-stimulating hormone were adjusted, as the American Association of Endocrinologists has recommended, around 20% of the population might be affected.[2]

Women are much more likely than men to suffer from either condition. Much of this is due to the effects of pregnancy. See also: thyroid disease in pregnancy.

Hyperthyroidism and Hypothyroidism In Animals

Animals may also suffer from an underactive or overactive thyroid. Hyperthyroidism is much more common in household pets, however, with about 2% of cats over 10 and 1-2% of dogs suffering from the disorder.[3][4]

References

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