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Hyperthyroidism

German: Hyperthyreose, Hyperthyreoidismus, Schilddrüsenüberfunktion

1 Definition

Hyperthyroidism means having abnormally high levels of thyroid hormones. Thyroid hormones, which are made by the thyroid gland in the lower front of the neck, regulate the body's energy. When levels of thyroid hormones are unusually high, the body burns energy faster and many vital functions speed up.

2 Thyroid hormones

The thyroid gland produces the hormones thyroxine (tetra-iodothyronine) and tri-iodothyronine, which set the body's metabolic rate. The more of these hormones in the blood, the faster the metabolism. The hormones are referred to as T4 and T3 respectively, reflecting the number of iodine atoms in each hormone. The body needs about 150µg of iodine a day to allow the thyroid to make sufficient hormones. The pituitary gland in the brain regulates the production of the hormones by releasing a chemical called thyroid stimulating hormone (TSH).

3 Anatomy and Physiology of Thyroid Gland

Thyroid gland is a small gland, normally weighing less than one ounce, located in the front of the neck. It is made up of two halves, called lobes that lie along the windpipe (trachea) and are joined together by a narrow band of thyroid tissue, known as the isthmus.

The thyroid is situated just below the "Adams apple" or larynx. During development (inside the womb) the thyroid gland originates in the back of the tongue, but it normally migrates to the front of the neck before birth. Sometimes it fails to migrate properly and is located high in the neck or even in the back of the tongue (lingual thyroid).This is very rare. At other times it may migrate too far and ends up in the chest (this is also rare).

The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Thyroid cells are the only cells in the body which can absorb iodine. These cells combine iodine and the amino acid tyrosine to make T3 and T4. T3 and T4 are then released into the blood stream and are transported throughout the body where they control metabolism (conversion of oxygen and calories to energy). Every cell in the body depends upon thyroid hormones for regulation of their metabolism. The normal thyroid gland produces about 80% T4 and about 20% T3, however, T3 possesses about four times the hormone "strength" as T4.

The thyroid gland is under the control of the pituitary gland, a small gland the size of a peanut at the base of the brain (shown here in orange). When the level of thyroid hormones (T3 & T4) drops too low, the pituitary gland produces Thyroid Stimulating Hormone (TSH) which stimulates the thyroid gland to produce more hormones. Under the influence of TSH, the thyroid will manufacture and secrete T3 and T4 thereby raising their blood levels. The pituitary senses this and responds by decreasing its TSH production. One can imagine the thyroid gland as a furnace and the pituitary gland as the thermostat. Thyroid hormones are like heat. When the heat gets back to the thermostat, it turns the thermostat off. As the room cools (the thyroid hormone levels drop), the thermostat turns back on (TSH increases) and the furnace produces more heat (thyroid hormones).

The pituitary gland itself is regulated by another gland, known as the hypothalamus. The hypothalamus is part of the brain and produces TSH Releasing Hormone (TRH) which tells the pituitary gland to stimulate the thyroid gland (release TSH). One might imagine the hypothalamus as the person who regulates the thermostat since it tells the pituitary gland at what level the thyroid should be set.

4 Epidemiology

Around one in 20 people will experience some form of thyroid dysfunction in their lifetime. Hyperthyroidism tends to affect women more than men. Around two in every 100 women will experience some degree of hyperthyroidism. The most common cause is Graves' disease, which is due to an immune system abnormality. Other causes of an overactive thyroid include local inflammation (thyroiditis), nodules or lumps. There is no cure for hyperthyroidism, but it can be successfully managed with treatments such as anti-thyroid medication.

5 Causes

Numerous factors have been identified for causing hyperthyroidism.

  • Graves' Disease

Graves' disease occurs when the immune or protective mechanisms of the body mistakenly activate the thyroid gland to release thyroid hormone. This is the primary cause of hyperthyroidism and usually causes protrusion of the eyes, redness on the front of the legs, and other side effects associated with hyperthyroidism as listed below.

  • Pituitary adenoma

A pituitary adenoma is a tumor that secretes a hormone that activates the thyroid gland above and beyond normal stimulation. This causes the release of thyroid hormone in excess and the signs and symptoms of hyperthyroidism.

  • Pituitary resistance

Normally, circulating thyroid hormone prevents the hypothalamus and pituitary from calling for unnecessary thyroid hormone, similar to the way a warm room prevents a thermostat from releasing more heat into the room. Sometimes the pituitary does not respond to the stimulation it receives from thyroid hormones and continues to release more TSH to the thyroid gland. More thyroid hormone is released which causes the signs and symptoms associated with hyperthyroidism.

  • Toxic adenoma

This is a type of tumor that is an abnormal extension of the thyroid gland which releases excess thyroid hormone. This tumor does not need releaser hormone from the pituitary so it continuously releases thyroid hormone on its own.

  • Multinodular goiters

Multinodular goiter disease, the second most common cause of hyperthyroidism, is characterized by soft nodules that grow slowly within the thyroid. They can produce thyroid hormone on their own to cause signs and symptoms of hyperthyroidism. Multinodular goiters are often asymptomatic, however, can grow large and compress the windpipe to cause difficulty breathing or swallowing.

  • Inflammatory thyroid disease

Sometimes the thyroid can become inflamed, usually from a virus or occasionally postpartum. Inflammation from a virus usually is painful and lasts from two to six months. The cause following pregnancy is poorly understood and can reoccur with subsequent pregnancies.

  • Ectopic thyroid tissue

Other tissues and organs of the body can abnormally produce another source of thyroid hormone. Examples are tumors in the ovary and follicular cancer.

  • Medication

Hypothyroid patients that are overtreated with thyroid medication can actually become hyperthyroid and display signs and symptoms of excess thyroid hormone. About 2% of patients on amiodarone will become hyperthyroid due to the drugs high concentration of iodine (37% by weight) in its molecular structure. This, through a number of steps, can ultimately cause an increased release of thyroid hormone into the blood stream.

6 Clinical Manifestations

The following are the most common symptoms of hyperthyroidism. However, each individual may experience symptoms differently. Symptoms may include:

  • General manifestations:
    • irritability
    • weak muscles, especially in the upper arms and thighs
    • shaky hands
  • Neurological manifestations:
    • Nervousness
    • sleeping difficulty
    • confusion
  • Dermatological manifestations:
    • increased perspiration ´
    • thinning of the skin
    • fine, brittle hair
  • Ophthamological manfestations:
  • Cardiovascular System:
  • Gastro-Intestinal manifestations:
    • increased bowel movements
    • weight loss
  • ReproductiveSystem:
    • Irregularmenstrual cycle
  • Lab Manifestations:
    • Anemia
    • Low cholesterol
    • High Calcium
    • High alkaline Phosphatase.

7 Diagnosis

Blood samples should be taken in order to determine the levels of TSH hormone (thyroid stimulating hormone) and the two metabolic hormones thyroxine (T4) and tri-iodothyronine (T3). TSH is produced by the pituitary gland in the brain and travels through the bloodstream to the thyroid gland. When there is too much T4 or T3 being released by the thyroid, the pituitary stops releasing TSH. In hyperthyroidism the concentration of TSH is, therefore, usually low while the concentration of T4 and T3 is high.

In order to determine the exact cause of the increased thyroid activity it may also be necessary to perform one or more medical imaging examinations such as an ultrasound examination of the gland.

8 Treatment

Treatment for hyperthyroidism is very specific for each patient. The goal of treatment is to restore the thyroid gland to normal function, producing normal levels of thyroid hormone. Specific treatment for hyperthyroidism will be determined based on:

  • type of hyperthyroidism
  • extent of the disease
  • tolerance of the patient for specific medications, procedures, or therapies
  • expectations for the course of the disease

The following are some possible treatments for hyperthyroidism:

  1. If a patient is diagnosed with thyroiditis, the treatment of choice is with beta blockers. Beta blockers don't reduce the thyroid hormone levels, but they may help the patient feel better if he/she has shakiness or a fast heartbeat. One may not be able to take a beta blocker if one has heart or lung problems.
  2. Radioactive iodine (RAI) is a common, safe and effective treatment for hyperthyroidism. This medicine is taken orally either dissolved in water or as a capsule. It helps reduce the size of the thyroid gland, as well as the symptoms of hyperthyroidism. One cannot take RAI if the patient is pregnant or breastfeeding. In case patient is thinking about conceiving, then she will need to wait 6 months after treatment with RAI. For 3 days after treatment with RAI, patient should double flush his toilet and wash his hands often. One should also avoid contact with young children and pregnant women during this time, to avoid harmful effects by radioactivity.
  3. Antithyroid drugs (ATDs) make the thyroid gland stop producing too many hormones. They include methimazole (brand name: Tapazole) and propylthiouracil. Methimazole is used most often because it is easier to take and causes fewer side effects. Propylthiouracil is safer if the patient is pregnant or breastfeeding. ATDs usually relieve the symptoms of hyperthyroidism in 6 to 8 weeks. Treatment with ATDs usually continues for a year or more, and there's a chance that hyperthyroidism may come back ones the treatment had stopped.
  4. Thyroidectomy is an option if the thyroid gland is very large and is compressing other parts of the neck.

9 Prevention

Hyperthyroidism caused by Graves' disease is a genetic disease that cannot be prevented. People who smoke are more likely to develop Graves' disease and Graves' ophthalmopathy than people who do not smoke.

10 References

http://www.healthsystem.virginia.edu/uvahealth/adult_endocrin/hypert.cfm

http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Thyroid_disorders_hyperthyroidism?OpenDocument

http://www.endocrineweb.com/thyfunction.html

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