Hypothyroidism (pronounced ) is a deficiency of thyroid hormone in humans and other vertebrates. It can be due to an abnormality in the thyroid gland, or less commonly, the pituitary gland or hypothalamus. It can result from a lack of a thyroid gland due to iodine-131, used to treat thyroid cancer and hyperthyroidism, its surgical removal , or its absence at birth. Cretinism is a form of hypothyroidism found in infants .

In adults, hypothyroidism is associated with the following symptoms:

Early

• Poor muscle tone ( muscle hypotonia)

• Fatigue

• Cold intolerance, increased sensitivity to cold

• Constipation

• Depression

• Muscle cramps and joint pain

• Goiter

• Thin, brittle fingernails

• Coarse hair

• Paleness

• Decreased sweating

• Dry, itchy skin

• Weight gain and water retention

• Bradycardia (low heart rate ??? fewer than sixty beats per minute)

Late

• Slow speech and a hoarse, breaking voice ??? deepening of the voice can also be noticed, caused by Reinke's Edema.

• Dry puffy skin, especially on the face

• Thinning of the outer third of the eyebrows ( sign of Hertoghe)

• Abnormal menstrual cycles

• Low basal body temperature

Uncommon

• Impaired memory

• Impaired cognitive function (brain fog) and inattentiveness.

• A slow heart rate with ECG changes including low voltage signals. Diminished cardiac output and decreased contractility

• Reactive (or post-prandial) hypoglycemia

• Sluggish reflexes

• Hair loss

• Anemia caused by impaired haemoglobin synthesis (decreased EPO levels), impaired intestinal iron and folate absorption or B₁₂ deficiency from pernicious anemia

• Difficulty swallowing

• Shortness of breath with a shallow and slow respiratory pattern

• Increased need for sleep

• Irritability and mood instability

• Yellowing of the skin due to impaired conversion of beta-carotene to vitamin A

• Impaired renal function with decreased glomerular filtration rate

• Elevated serum cholesterol

• Acute psychosis ( myxedema madness) (a rare presentation of hypothyroidism)

• Decreased libido due to impairment of testicular testosterone synthesis

• Decreased sense of taste and smell ( anosmia)

• Puffy face, hands and feet (late, less common symptoms)

• Gynecomastia

• Deafness

About three percent of the general population is hypothyroid. In iodine-replete individuals hypothyroidism is generally caused by Hashimoto's thyroiditis, or otherwise as a result of either an absent thyroid gland or a deficiency in stimulating hormones from the hypothalamus or pituitary.

Hypothyroidism can result from postpartum thyroiditis, a condition that affects about 5% of all women within a year of giving birth. The first phase is typically hyperthyroidism; the thyroid then 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 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 iodide is a substrate for thyroid hormones, high levels reduce iodide organification in the thyroid gland, decreasing hormone production. The antiarrhythmic agent amiodarone can cause hyper- or hypothyroidism due to its high iodine content.

Hypothyroidism can be caused by lithium -based mood stabilizers, usually used to treat bipolar disorder (previously known as manic depression).

Hypothyroidism is often classified by association with the indicated organ dysfunction (see below):

Origin || Description
Primary	Thyroid gland	The most common forms include Hashimoto's thyroiditis (an autoimmune disease) and radioiodine therapy for hyperthyroidism.
Secondary	Pituitary gland	Occurs if the pituitary gland does not create enough thyroid-stimulating hormone (TSH) to induce the thyroid gland to produce enough thyroxine and triiodothyronine. Although not every case of secondary hypothyroidism has a clear-cut cause, it is usually caused by damage to the pituitary gland, as by a tumor, radiation, or surgery.
Tertiary	Hypothalamus	Results when the hypothalamus fails to produce sufficient thyrotropin-releasing hormone (TRH). TRH prompts the pituitary gland to produce thyroid-stimulating hormone (TSH). Hence may also be termed ''hypothalamic-pituitary-axis hypothyroidism.''

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 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

• Basal body temperature

Hypothyroidism is treated with the levorotatory forms of thyroxine ( levothyroxine) (L-T₄) and triiodothyronine (liothyronine) (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 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

A 2000 paper suggested "clear improvements in both cognition and mood" from combination therapy,

and another in 2001 concluded that combined treatment seemed to be more effective than treatment with T4 alone on eight main symptoms of hypothyroidism.  

However, more recent studies have shown no improvement in mood or mental abilities for those on combination therapy, and possibly impaired well-being from subclinical hyperthyroidism..

In addition, a metaanalysis of 11 randomized controlled trials which looked at a wider range of symptoms including: bodily pain, anxiety, fatigue, body weight and a range of other factors, found no difference between the combined treatment and therapy with T4 alone.

There is also concern among some practitioners about the use of T3 due to its short half-life. T3 when used on its own as a treatment results in wide fluctuations across the course of a day in the thyroid hormone levels, and with combined T3/T4 therapy there continues to be wide variation throughout each day.

Subclinical hypothyroidism

Subclinical hypothyroidism occurs when thyrotropin (TSH) levels are elevated but thyroxine (T₄) and triiodothyronine (T₃) levels are normal.

A more recent metanalysis looking into whether subclinical hypothyroidism may increase the risk of cardiovascular disease, as has been previously suggested, found a possible modest increase and suggested further studies be undertaken with coronary heart disease as an end point "before current recommendations are updated."

Alternative treatments

Alternative practitioners may combine conventional serum tests with less conventional tests to assess thyroid hormone function, or simply look at symptoms. Compounded slow release T3 has been suggested for use in combination with T4, which proponents argue will mitigate many of the symptoms of functional hypothyroidism and improve quality of life. This is still controversial and is rejected by the conventional medical establishment .

• Subacute lymphocytic thyroiditis

• Hyperthyroidism

• Risk factors in pregnancy

• Hypothyroidism Booklet - American Thyroid Association