Hypothyroidism is the disease state in humans and animals caused by insufficient production of thyroid hormone by the thyroid gland.

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 worldwide. 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. It is a relatively common disease in purebred domestic dogs as well, and can have a hereditary basis in dogs.^{[citation needed]}

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

Hypothyroidism is often classified by the organ of origin:^[1][2]

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:^[4]

• atypical depression (which may present as dysthymia)
• bipolar spectrum syndrome (including bipolar I or bipolar II disorder, cyclothymia, or premenstrual syndrome)
• borderline personality disorder^{[citation needed]}
• 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^[5] 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:^[3]

Early symptoms

^[6]

• 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

^[6]

• 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
• 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^[7]
• 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 Association of Clinical Endocrinologists recommends the use of levothyroxine (T4) as thyroid replacement. In general, desiccated thyroid hormone, combinations of thyroid hormone, or triiodothyronine should not be used for replacement therapy.^[8]

The American Thyroid Association cautions against taking herbal remedies, and warns that taking too much iodine can actually worsen hypothyroidism.^[3]

Subclinical hypothyroidism

A meta-analysis by the Cochrane Collaboration found no benefit except "some parameters of lipid profiles and left ventricular function".^[9]

References

[www.aace.com/pub/pdf/guidelines/hypo_hyper.pdf]