Hypothyroidism

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Hypothyroidism
Triiodothyronine, the most active form of thyroid hormone
ICD-10	E03.9
ICD-9	244.9
DiseasesDB	6558
eMedicine	med/1145
MeSH	D007037

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Overview

Hypothyroidism is the disease state in human 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 Hashimoto's thyroiditis, or by a lack of the thyroid gland or a deficiency of hormones from either the hypothalamus or the pituitary.

Hypothyroidism can result from postpartum thyroiditis, a condition that affects about 5% of all women within a year after giving birth. The first phase is typically hyperthyroidism. Then, the thyroid either returns to normal or a woman develops hypothyroidism. Of those women who experience hypothyroidism associated with postpartum thyroiditis, one in five will develop permanent hypothyroidism requiring life-long treatment.

Hypothyroidism can also result from sporadic inheritance, sometimes autosomal recessive.

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.

Complete Differential Diagnosis of Causes of Hypothyroidism

• Amyloidosis
• Antithyroid drugs
• Cystinosis
• Dermatosclerosis
• Drugs
• Hashimoto's Thyroiditis
• Head and neck surgery
• Hemochromatosis
• Iodine deficiency
• Neoplastic
• Peripheral thyroid gland disorder resistance
• Radiation therapy to head neck or chest area
• Radioactive iodine therapy
• Reidel's Thyroiditis
• Sarcoidosis, granulomatous disease
• Scleroderma
• Secondary hypothyroidism
• Silent thyroiditis
• Subacute thyroiditis
• Subtotal thyroidectomy
• Supprative thyroiditis
• Thyreostatic therapy
• Thyroid agenesis, dysgenesis, ectopy
• TSH (thyroid stimulating hormone) deficiency

General psychological associations

Hypothyroidism can be caused by lithium-based mood stabilizers, usually 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 I or bipolar II disorder, cyclothymia, or premenstrual syndrome)
• borderline personality disorder
• a psychotic disorder (typically, paranoid schizophrenia)
• inattentive ADHD or sluggish cognitive tempo

Symptoms

The ability of Hypothyroidism to mimic a number of medical conditions originates in the vast functions of the thyroid hormones, which are reduced or 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 a few. Thus, when the thyroid hormone content gets out of balance, systems covering the whole body are 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
• Cold intolerance, increased sensitivity to cold
• Constipation
• Weight gain
• 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
• Low basal body temperature

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.
• Reactive (or post-prandial) hypoglycemia
• 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 if present at birth, and untreated.

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 blood testing if the TSH is normal and hypothyroidism is still suspected:

• 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
• testing for anemia, including ferritin

Treatment

Main article: Medical use of thyroid hormones

Both synthetic and animal-derived thyroid tablets are available and can be prescribed for patients in need of additional thyroid hormone. Thyroid hormone is 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 hypothyroidism.

Some researchers would suggest that there is no benefit to treatment with the T₄ - T₃ combination therapy over the T₄ monotherapy. A meta-analysis of 11 randomized controlled trials found no advantage with the T₄ - T₃ combination therapy over the T₄ monotherapy.

However, many of the studies involved in this meta-analysis were not properly designed. They tested the medication's effectiveness by providing the same dose of thyroid medication to each patient and by utilizing the TSH test as a measure of treatment effectiveness. Thyroid medication must be individualized to the patient, carefully observing free T4 and free T3 (not TSH, which is a diagnostic tool but not a treatment tool). Beneficial effects would not expect to be seen at doses not tailored to the individual's needs. Clinically, most physicians prescribing T3 as part or all of a hypothyroid patient's treatment find that most patients receiving T3 in fact must be on a larger dose than was administered in these studies. The meta-analysis results ought to be held in question, therefore; and the knowledgeable physician will administer T3 medication for the hypothyroid patient who continues to present with symptoms and whose free T3 level is low, or low in the range.

Subclinical hypothyroidism

A meta-analysis by the Cochrane Collaboration found no benefit except "some parameters of lipid profiles and left ventricular function".

External links

• Thyroid Section of The Hormone Foundation
• The Endocrine Society
• 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
• Mayo Clinic: Hypothyroidism

References

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