Hyperthyroidism

Hyperthyroidism is an endocrine disorder resulting from an excessive amount of thyroid hormone in the blood. This condition is usually due to an overactive thyroid gland that release too much thyroid hormones.

Thyroid hormones are produced in the thyroid gland. The thyroid gland is located in the front part of the neck, under the Adam's apple. It has a butterfly shape and wrapped around the windpipe (trachea). The thyroid gland has two similar-sized lobes joined by an isthmus across the front of the trachea.

Thyroid hormones are essential for the cells. They play an important role in growth and development, heart rate, blood pressure, body temperature, and metabolic rate (the rate at which food is converted into energy in the body cells).

The thyroid gland uses iodine to produce thyroid hormones.

The 2 most important thyroid hormones are:

• Thyroxine (T4)
• Triiodothyronine (T3).

The T4 hormone contains 4 iodine molecules, while the T3 hormone only contains 3 iodine molecules.

The iodine needed to produce thyroid hormones is found in fish and mollusks, bread, and iodized salt.

More than 99% of all thyroid hormones are bound to proteins in the blood and are inactive, meaning they cannot interact with the body's cells. Only a small fraction of circulating thyroid hormones are free, unbound to proteins. This small fraction of thyroid hormones is essential for the regulation of cellular metabolism.

The production of thyroid hormones is controlled by the release of a hormone called TSH (Thyroid Stimulating Hormone) produced by the pituitary gland or hypophysis. This gland is located at the base of the brain.

When there is an excess of thyroid hormones in the blood, the pituitary gland stops the secretion of TSH, and vice versa. This helps maintain a relatively constant level of circulating thyroid hormones in the blood.

Sometimes, the medical term thyrotoxicosis is used as a synonym for hyperthyroidism but they are not exactly the same. Thyrotoxicosis refers to the symptoms caused by the excessive amount of thyroid hormones in tissues and organs. Thyrotoxicosis is usually the consequence of hyperthyroidism.

When a person suffers from hyperthyroidism, the concentration of thyroid hormones T3 and T4 in the body increases.

However, depending on the reason of the increased production of thyroid hormones, hyperthyroidism can be classified into:

• Primary hyperthyroidism: It is due to an excessive production of thyroid hormones by the thyroid gland. TSH level in the blood will be below the normal range. This is the most common cause.
• Secondary or central hyperthyroidism: It is due to an excessive production of TSH by the pituitary gland or hypophysis. The TSH level in the blood will be high. It is less frequent.

The main underlying causes of hyperthyroidism are:

• Graves disease (Diffuse Toxic Goiter):
  It accounts for 60 to 90% of hyperthyroidism cases. There is an enlarged thyroid gland of the diffuse type. The disease is characterized by the triad of:
  • Hyperthyroidism
  • Bulging eyes (exophthalmos)
  • Graves dermopathy (skin rash on the shins and tops of the feet)
  The diagnosis may be confirmed by a thyroid scintigraphy and blood test to quantify the thyroid hormones in the blood, including abnormally high levels of the TSI (Thyroid Stimulating Immunoglobulin) antibody. [Not to be confused with the pituitary hormone "thyroid stimulating hormone" or TSH, which level is always low in Graves disease.] Exophthalmos may precede or occur simultaneously to the hyperthyroidism and it may lead to visual impairment. Graves disease has a family predisposition. Risk factors that can trigger the disease include stress, tobacco, radiation to the neck, drugs (such as interleukin-2 and interferon-alpha), and some viruses.
  
• Toxic thyroid nodules:
  The thyroid gland sometimes contains nodules. A nodule can become overactive and produce an excessive amount of thyroid hormones. If there is a single nodule, it is called a toxic adenoma; if the overactive nodules are numerous, it is called a toxic multinodular goiter.
• Thyroid hormones:
  An overdose of thyroid hormones can cause hyperthyroidism.
• Iodine:
  The thyroid gland uses iodine to produce thyroid hormones. An excess intake of iodine can cause hyperthyroidism, especially if the patient already suffered from a previous thyroid disease such as goiter (abnormal enlargement of the thyroid). Iodine is found in large amounts in some medication, such as which is used to treat ventricular arrhythmias.
• Thyroiditis:
  Inflammation of the thyroid gland that may occur after pregnancy or after a viral infection. In both cases, it can lead to a transient hyperthyroidism state. After that, the patient usually develops hypothyroidism (decreased production of thyroid hormones).

It affects approximately around 1% of the general population. However, it is much more common in women than in men with a female-to-male ratio of 5 to 1.

Patients with mild hyperthyroidism may be asymptomatic. Symptoms become more evident as the disease progresses and they are usually related to an increase in the metabolism.

The most common symptoms are:

• Anxiety
• Tremors
• Weight loss despite an increase in appetite
• Sweating
• Palpitations
• Heat intolerance
• Tendency to diarrhea

Other symptoms of hyperthyroidism include:

• Fatigue
• Weakness
• Insomnia
• Hair loss
• Menstrual disorders in women

Hyperthyroidism can be successfully treated. However, if left untreated, it can lead to serious heart disease and a very serious complication called thyrotoxicosis crisis or thyroid storm.

The 3 main treatments of hyperthyroidism are surgery, medication and radioiodine.

• Surgery:
  Total or partial surgical removal of the thyroid gland (called thyroidectomy) eliminates the source of thyroid hormone production, in order to achieve a normal level of thyroid hormones in the blood. However, it may lead to some complications:
  • Hypothyroidism, or an underactive thyroid gland
  • Vocal cord (fold) paralysis
  • Incidental parathyroidectomy: Unintentional removal of the parathyroid glands located behind the thyroid gland. This leads to a decreased level of calcium in the blood (the parathyroid glands regulate calcium levels).
• Radioiodine (Radioactive iodine):
  It can be administered as a single capsule. It directly radiates the thyroid tissues, destroying (ablating) them. After the treatment, the patient recovers a normal thyroid functionality in 8 to 12 weeks. It is contraindicated during pregnancy and lactation.
• Medication:
  Antithyroid medication prevents an excessive production of thyroid hormones by the thyroid gland. The most commonly used drugs are methimazole, carbimazole, and propylthiouracil. Beta-blockers (such as propranolol) are also used.

The treatment of choice for Graves disease is radioactive iodine, with surgery reserved only for special cases, such as pregnant women. A toxic adenoma is usually treated with surgery. Toxic multinodular goiter can be treated with both options. In all cases, antithyroid medication is needed temporarily or permanently.

Hyperthyroidism related to thyroiditis is a transient problem and treated differently, usually with beta-blockers and anti-inflammatory drugs.