hypothyroidism n : an underactive thyroid gland; a glandular disorder resulting from insufficient production of thyroid hormones [ant: hyperthyroidism]

English

Noun

hypothyroidism

1. The disease state caused by insufficient production of thyroid hormone by the thyroid gland.

Translations

• Icelandic: slímbjúgur

Hypothyroidism is the disease state in humans and animals caused by insufficient production of thyroid hormone by the thyroid gland. Cretinism is a form of hypothyroidism found in infants.

Causes

About three percent of the general population is hypothyroid. Factors such as iodine deficiency or exposure to I-131 can increase that risk. There are a number of causes for overt hypothyroidism. Historically, and still in many developing countries, iodine deficiency is the most common cause of hypothyroidism worldwide. In iodine-replete individuals, 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.

Hypothyroidism is also a relatively common hormone disease in domestic dogs, with some specific breeds having a definite predisposition.

Temporary hypothyroidism can be due to the Wolff-Chaikoff effect. A very high intake of iodine can be used to temporarily treat hyperthyroidism, especially in an emergency situation. Although iodine is substrate for thyroid hormones, high levels prompt the thyroid gland to take in less of the iodine that is eaten, reducing hormone production.

Hypothyroidism is often classified by the organ of origin:

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. This is 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
• Depression
• Constipation
• Muscle cramps and joint pain
• Arthritis
• Goiter
• Thin, brittle fingernails
• Thin, brittle hair
• Paleness
• Dry, itchy skin
• Weight gain
• Bradycardia (low HR <60 BPM)

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)
• Depression

Pediatric

Hypothyroidism in pediatric patients can cause the following additional symptoms:

• short stature
• mental retardation if present at birth, and untreated.
• Previously called Cretinism

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 by the pituitary gland. 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 (T3)). 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:

• free triiodothyronine (fT3)
• free levothyroxine (fT4)
• total T3
• total T4

Additionally, the following measurements may be needed:

• 24 hour urine free T3
• 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

Hypothyroidism is treated with the levorotatory forms of thyroxine (L-T4) and triiodothyronine (L-T3). 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 dosing. There are several different treatment protocols in thyroid replacement therapy:

T4 Only

This treatment protocol involves supplementation of levothyroxine alone, in a synthetic form. It is currently the standard treatment in mainstream medicine.

T4 and T3 in Combination

This treatment protocol involves administering both synthetic L-T4 and L-T3 simultaneously in combination.

Desiccated Thyroid Extract

Desiccated thyroid extract is an animal based thyroid extract, most commonly from a porcine source. It is also a combination therapy, containing natural forms of L-T4 and L-T3.

Treatment Controversy

Though the current standard treatment in thyroid therapy is levothyroxine only, there exists some controversy about which treatment protocol is most effective. The American Association of Clinical Endocrinologists (AACE) states that desiccated thyroid hormone, combinations of thyroid hormone, or triiodothyronine should not generally be used for replacement therapy. However, the medical journal Thyroid Science claims that, "today's conventional thyroid hormone therapy, T4-replacement, has been documented to be ineffective and harmful to many patients". Other recent publications have also challenged the status quo, showing that combination therapies can be more effective.

Subclinical hypothyroidism

Subclinical hypothyroidism occurs when thyrotropin (TSH) levels are elevated but thyroxine (T4) and triiodothyronine (T3) levels are normal. In primary hypothyroidism, TSH levels are high and T4 and T3 levels are low. Endocrinologists are puzzled because TSH usually increases when T4 and T3 levels drop. TSH prompts the thyroid gland to make more hormone. Endocrinologists are unsure how subclinical hypothyroidism affects cellular metabolic rates (and ultimately the body's organs) because the levels of the active hormones are adequate. Some have proposed treating subclinical hypothyroidism with levothyroxine, the typical treatment for overt hypothyroidism, but the benefits and the risks are unclear. Reference ranges have been debated as well. The American Association of Clinical Endocrinologists (ACEE) supports a narrower TSH range, especially when the person has clinical signs of thyroid disease. This reference range may reduce the risks of goiter, thyroid nodules, thyroid cancer, and overt hypothyroidism, but remains controversial. There is always the risk of overtreatment and hyperthyroidism. Some studies have suggested that subclinical hypothyroidism does not need to be treated. A meta-analysis by the Cochrane Collaboration found no benefit of thyroid hormone replacement except "some parameters of lipid profiles and left ventricular function".

References

External links

• Thyroid Section of The Hormone Foundation
• Thyroid Science journal
• The Endocrine Society
• American Association of Clinical Endocrinologists
• American Foundation of Thyroid Patients
• Thyroid Foundation of America
• Thyroid Australia
• Mayo Clinic: Hypothyroidism
• AHSTA: Hypothyroidism in Thyroid Autoimmunity