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Hypothyroidism is the disease state in humans and animals caused by insufficient production of thyroid hormone by the thyroid gland. There are several distinct causes for chronic hypothyroidism in human beings, the most common being Hashimoto's thyroiditis (an autoimmune disease) and radioiodine therapy for hyperthyroidism. Historically, iodine deficiency was the most common cause of hypothyroidism world-wide. The disease may also be caused by a lack of thyroid gland or a deficiency of hormones from either the hypothalamus or the pituitary.
Secondary hypothyroidism 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, results when the hypothalamus fails to instruct the pituitary to produce sufficient TSH.
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.
It is claimed by some in the water fluoridation controversy that hypothyroidism is related to fluoride.
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.
Signs and symptoms
Hypothyroidism has been called the "great imitator" for the vast number of medical conditions it can mimic:
- Thyroid hormones are essential and primary regulators of the body's metabolism. Imbalances can affect virtually every metabolic process in the body, exerting significant effects on mood and energy level. Thyroid function has a profound impact on overall health via its modulation of carbohydrate, protein, and fat metabolism, vitamin utilization, mitochondrial function, digestive process, muscle and nerve activity, blood flow, oxygen utilization, hormone secretion, sexual and reproductive health, and many other physiological parameters. 
In addition, patients with hypothyroidism and psychiatric symptoms may be diagnosed with:
- atypical depression (which may present as dysthymia)
- bipolar spectrum syndrome (including manic-depression, mixed mania, bipolar depression, rapid-cycling bipolar disorder, cyclothymia, and premenstrual syndromes)
- borderline personality disorder
- psychotic disorder (typically, paranoid psychosis)
- Slowed speech and a hoarse, breaking voice. Deepening of the voice can also be noticed.
- Impaired memory
- Impaired cognitive function (brain fog)
- Urticaria (hives)
- Migraine headache
- Increased sensitivity to heat and cold
- A slow heart rate with ECG changes including low voltage signals. Diminished cardiac output and decreased contractility.
- Pericardial effusions may occur.
- Sluggish reflexes
- Dry puffy skin, especially on the face, and hair loss, especially thinning of the outer third of the eyebrows
- Depression (especially in the elderly)
- Mild weight gain
- Anemia caused by impaired hemoglobin synthesis (decreased EPO levels), impaired intestinal iron and folate absorption or B12 deficiency from pernicious anemia
- Slowed metabolism
- Fatigue (physical)
- anxiety/panic attacks
- Choking sensation or difficulty swallowing
- Shortness of breath with a shallow and slow respiratory pattern.
- Impaired ventilatory responses to hypercapnia and hypoxia.
- Increased need for sleep
- Muscle cramps and joint pain
- Brittle fingernails
- Osteopenia or Osteoporosis
- Yellowing of the skin due to impaired conversion of beta-carotene to vitamin A
- Abnormal menstrual cycles
- Impaired renal function with decreased GFR.
- Thin, fragile or absent cuticles
- Infertility or difficulty becoming pregnant
- Elevated serum cholesterol
- Mood instability
- Acute psychosis (myxedema madness) is a rare presentation of hypothyroidism
- Poor muscle tone (muscle hypotonia)
- Decreased libido
Hypothyroidism in pediatric patients can cause the following additional symptoms:
- short stature
- mental retardation
Hypothyroidism in domestic dogs
Hypothyroidism in dogs is a slowly progressive disease as well and can be difficult to diagnose accurately in the early stages. Dogs with hypothyroidism are plagued by a number of health problems including recurring skin infections and lethargic behavior. Hypothyroidism in dogs may also progress to myxedema coma. It may also cause aggression in affected dogs. Dogs may show sudden weight gain, and hair loss at the base of the tail.
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
- Main article: Thyroid hormone#Medical use of thyroid hormones
A doctor can perform a blood test to see if a patient suffers from hypothyroidism. A synthetic thyroid supplement can be prescribed, which the patient must take daily to replace the missing hormone. The American Thyroid Association cautions against taking herbal remedies, and warns that taking too much iodine can actually worsen both hypothyroidism and hyperthyroidism. (See ATA Hypothyroidism Booklet, linked below.)
Patients with psychiatric symptoms (mood or psychosis) and physical symptoms of hypothyroidism, with subclinical hypothyroidism (normal TSH but low free T3) are often treated with thyroid supplement to bring the free T3 levels into the upper range of normal.
- ATA Hypothyroidism Booklet
- American Thyroid Association
- American Association of Clinical Endocrinologists
- American Foundation of Thyroid Patients
- Thyroid Foundation of America
- ↑ ATA Hypothryoidism Booklet, pge 6, American Thyroid Association, 2003.
- ↑ Comprehensive Thyroid Assessment, Geneva Diagnostics web site article
- ↑ Current Psychiatry Online, 
- ↑ http://www.thyroid.org/patients/brochures/Hypothyroidism%20_web_booklet.pdf#search=%22hypothyroidism%22
Endocrine pathology of psychological interest (E00-35)
thyroid Hypothyroidism (Iodine deficiency, Cretinism, Congenital hypothyroidism, Goitre) - Hyperthyroidism (Graves-Basedow disease, Toxic multinodular goitre) - Thyroiditis (De Quervain's thyroiditis, Hashimoto's thyroiditis)
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