Hypothyroidism

Hypothyroidism
Specialty	Endocrinology

Hypothyroidism is the disease state in humans and animals caused by insufficient production of thyroid hormone by the thyroid gland. It affects about 5% of the United Kingdom population over 60 years of age. As of 2006, more than 1% of the United Kingdom population were receiving T₄ (Thyroxine) replacement therapy for hypothyroidism.

Causes

There are several distinct causes for chronic hypothyroidism. Historically and, still, in many developing countries iodine deficiency is the most common cause of hypothyroidism world-wide. In present day developed countries, however, hypothyroidism is mostly caused by a lack of the thyroid gland or a deficiency of hormones from either the hypothalamus or the pituitary.

Hypothyroidism can also result from sporadic inheritance, sometimes autosomal recessive. It is a relatively common disease in purebred domestic dogs as well, and can have a hereditary basis in dogs.

Temporary hypothyroidism can be due to the Wolff-Chaikoff effect.

Hypothyroidism is often classified by the organ of origin:

Type	Origin	Description
Primary hypothyroidism	thyroid gland	The most common forms include Hashimoto's thyroiditis (an autoimmune disease) and radioiodine therapy for hyperthyroidism.
Secondary hypothyroidism	pituitary gland	Occurs if the pituitary gland does not create enough thyroid stimulating hormone (TSH) to induce the thyroid gland to create a sufficient quantity of thyroxine. Although not every case of secondary hypothyroidism has a clear-cut cause, it is usually caused when the pituitary is damaged by a tumor, radiation, or surgery so that it is no longer able to instruct the thyroid to make enough hormone
Tertiary hypothyroidism, also called hypothalamic-pituitary-axis hypothyroidism	hypothalamus	Results when the hypothalamus fails to produce sufficient TRH.

General psychological associations

Hypothyroidism can be caused by the medication Lithium which is used to treat Bipolar Disorder (previously known as Manic Depression).

In addition, patients with hypothyroidism and psychiatric symptoms may be diagnosed with:

• atypical depression (which may present as dysthymia)
• bipolar spectrum syndrome (including bipolar depression, manic-depression, mixed mania, rapid-cycling bipolar disorder, cyclothymia, and premenstrual syndromes)
• borderline personality disorder
• psychotic disorder (typically, paranoid psychosis)
• inattentive ADHD or more specifically sluggish cognitive tempo

Symptoms

Hypothyroidism has been called the "great imitator" for the vast number of medical conditions it can mimic. This ability originates in the vast functions of the thyroid hormones, which are absent in this case. The functions of thyroid hormones include modulation of carbohydrate, protein and fat metabolism, vitamin utilization, mitochondrial function, digestive process, muscle and nerve activity, blood flow, oxygen utilization, hormone secretion and sexual and reproductive health to mention some. Thus, when the thyroid hormone content gets out of balance, systems covering the whole body is affected, and that's why hypothyroidism can look like other diseases.

Conversely, sometimes other conditions can be mistaken for Hypothyroidism.

Adults

In adults, hypothyroidism is associated with the following symptoms:

Early symptoms

• Poor muscle tone (muscle hypotonia)
• Fatigue (physical)
• Cold intolerance, increased sensitivity to cold
• Constipation
• Weight gain
• Depression (especially in the elderly)
• Muscle cramps and joint pain
• Thin, Brittle fingernails
• Thin, brittle hair
• Paleness

Late symptoms

• Slowed speech and a hoarse, breaking voice. Deepening of the voice can also be noticed.
• Dry puffy skin, especially on the face
• Thinning of the outer third of the eyebrows
• Abnormal menstrual cycles (Specifically Menorrhagia)

Less common symptoms

• Heat intolerance, increased sensitivity to heat
• Impaired memory
• Impaired cognitive function (brain fog) and inattentiveness
• Urticaria (hives)
• Migraine headache
• A slow heart rate with ECG changes including low voltage signals. Diminished cardiac output and decreased contractility.
• Pericardial effusions may occur.
• Sluggish reflexes
• hair loss
• Anemia caused by impaired hemoglobin synthesis (decreased EPO levels), impaired intestinal iron and folate absorption or B12 deficiency from pernicious anemia
• anxiety/panic attacks
• difficulty swallowing
• Shortness of breath with a shallow and slow respiratory pattern.
• Impaired ventilatory responses to hypercapnia and hypoxia.
• Increased need for sleep
• Osteopenia or Osteoporosis
• Irritability and mood instability
• Yellowing of the skin due to impaired conversion of beta-carotene to vitamin A
• Impaired renal function with decreased GFR.
• Thin, fragile or absent cuticles
• Elevated serum cholesterol
• Acute psychosis (myxedema madness) is a rare presentation of hypothyroidism
• Decreased libido
• Decreased sense of taste and smell (late, less common symptoms)
• Puffy face, hands and feet (late, less common symptoms)

Pediatric

Hypothyroidism in pediatric patients can cause the following additional symptoms:

• short stature
• mental retardation

Severity

The severity of hypothyroidism varies widely. Some have few overt symptoms, others with moderate symptoms can be mistaken for having other diseases and states. Advanced hypothyroidism may cause severe complications including cardiovasular and psychiatric myxedema.

Diagnostic testing

To diagnose primary hypothyroidism, many doctors simply measure the amount of Thyroid-stimulating hormone (TSH) being produced. High levels of TSH indicate that the thyroid is not producing sufficient levels of Thyroid hormone (mainly as thyroxine (T4) and smaller amounts of triiodothyronine (fT3)). However, measuring just TSH fails to diagnose secondary and tertiary forms of hypothyroidism, thus leading to the following suggested minimum blood testing:

• thyroid-stimulating hormone (TSH)
• free triiodothyronine (fT3)
• free levothyroxine (fT4)
• total T3
• total T4

Additionally, the following measurements may be needed:

• antithyroid antibodies - for evidence of autoimmune diseases that may be damaging the thyroid gland
• serum cholesterol - which may be elevated in hypothyroidism
• prolactin - as a widely available test of pituitary function

Treatment

Main article: Medical use of thyroid hormones

A doctor can perform blood tests to see if a patient suffers from hypothyroidism. Both synthetic and animal-derived thyroid tablets are available and can be prescribed for patients in need of additional thyroid hormone. Thyroid hormone is usually taken daily, and doctors can monitor blood levels to help assure proper dosaging. The American Thyroid Association cautions against taking herbal remedies, and warns that taking too much iodine can actually worsen both hypothyroidism and hyperthyroidism.

A great deal of debate has emerged between treatment with T₄ (Thyroxin) monotherapy versus T₄ - T₃ (Thyroxine-Triiodothyronine) combination therapy. In July 2006 an evaluation of 11 published randomized clinical trials was published concluding no benefit to treatment with the T₄ - T₃ combination therapy over the T₄ monotherapy.

External links

• American Association of Clinical Endocrinologists
• American Foundation of Thyroid Patients
• Thyroid Foundation of America
• http://www.veterinarypartner.com/Content.plx?P=A&A=461
• Thyroid Australia

References

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