Hypothyroidism is the disease state caused by insufficient production of thyroid hormone by the thyroid gland. There are several distinct causes for chronic hypothyroidism, the most common being Hashimoto's thyroiditis and hypothyroidism following radioiodine therapy for hyperthyroidism.

The severity of hypothyroidism varies widely. Patients are classified as "subclinical hypothyroid" if diagnostic findings show thyroid hormone abnormalities, but they do not exhibit any symptoms. Others have moderate symptoms that can be mistaken for other diseases and states. Advanced hypothyroidism may cause severe complications, the most serious one of which is myxedema.

Signs and symptoms

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Adults

• 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 1/3 of the eyebrows
• Depression (especially in the elderly)
• Weight gain and obesity
• Anemia caused by impaired hemoglobin synthesis (decreased EPO levels), impaired intestinal iron and folate absorption or B12 deficiency from pernicious anemia
• Slowed metabolism
• Constipation
• Fatigue (physical)
• 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
• Decreased sex drive
• Brittle fingernails
• Osteoporosis
• Paleness
• Irritability
• 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
• Acute psychosis (myxedema madness) is a rare presentation of hypothyroidism
• Poor muscle tone (muscle hypotonia)

Children

Very Early Infancy

• Feeding problems
• Constipation
• Hoarseness
• Excessive sleepiness
• Protruding tongue
• Puffy appearance of hands and feet
• Deaf mutism

Later Infancy/Toddlerhood

• Protruding abdomen
• Rough, dry skin
• Delayed teething

After Toddlerhood

• Lack of normal growth
• Abnormally short for age on height/weight charts
• Puffy, bloated appearance
• Below-normal intelligence for age

Causes

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Neonatal hypothyroidism

Thyroid hormone is very important to neural development in the neonatal period. A deficiency of thyroid hormones can lead to cretinism. For this reason it is important to detect and treat thyroid deficiency early. In Australia, the Netherlands, and many other countries this is done by testing for TSH on the routine neonatal heel pricks performed by law on all newborn babies.

Hashimoto's thyroiditis

Sometimes called Hashimoto's Disease, this is part of the spectrum of autoimmune diseases and is related to Graves' disease, lymphocytic thyroiditis, and other organ-related autoimmune conditions such as Addison's disease, diabetes, premature menopause and vitiligo. Hashimoto's is a lymphocytic and plasmacytic thyroid inflammation that eventually destroys the thyroid. Patients require permanent thyroid hormone replacement.

Autoimmune hypothyroidism may also be part of a spectrum of disorders referred to as Schmidt's syndrome:

• Hypothyroidism
• Pernicious anemia
• Diabetes mellitus
• Adrenal insufficiency

Thyroid surgery for this has generally been a sub-total thyroidectomy. A large reason for this is the risk of destroying the parathyroids in a total thyroidectomy. If insufficient thyroid tissue remains to produce normal requirements then supplementary thyroxine is required.

Pituitary failure

Reduction or loss of TSH secretion by the pituitary is a rare cause of hypothyroidism. This constellation is usually referred to as "secondary hypothyroidism". Even rarer is tertiary hypothyroidism that is caused either by hypothalamic lesions or by interruption of signal transfer in the portal veins connecting the hypothalamus to the pituitary gland (Pickardt syndrome).

Iatrogenic

Hypothyroidism may occur as an adverse reaction to lithium used in the treatment of mood disorders, and in response to interferon and IL-2 treatment (e.g. for cancer). It may also be a result of the antiarrhythmic amiodarone.

Iodine deficiency

Severe iodine deficiency is another major cause of hypothyroidism. In areas of the world where there is an iodine deficiency in the diet, severe hypothyroidism can be seen in 5 to 15% of the population. In many countries, iodine deficiency is very rare due to the small amount of iodine salt that is added to common table salt.

Surgery on the thyroid is generally done in a form that allows some hormone-producing tissue to remain. Nevertheless, some patients will need hormone supplementation after surgery.

Treatment

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Myxedema coma

Myxedema coma is a medical emergency. The major imbalances are hypoglycemia, hyponatremia, hypothermia and acute renal failure. The initial management includes warming the patient, monitoring the vitals. Parenteral steroids is the initial drug (injection hydrocortisone 100 mg - 200 mg) given. Levothyroxine 600 micrograms is given through nasogastric tube or parenteral route.

Hypothyroidism

Clinically apparent hypothyroidism usually warrants treatment. In case the hypothyroidism is due to dietary minerals and iodine, supplementation with these may obviate the need for hormonal treatment, but only if iodine deficiency has been documented, which is very rare in the Western world. Whilst subclinical hypothyroidism is thought to have long-term consequences, such as atherosclerosis and coronary heart disease risks, there is no current consensus as to the benefits of treatment in this group.

Treatment is usually with levothyroxine (starting at 50 μg/day), a synthetic thyroxine analogue. There is no evidence suggesting that there is any need to substitute T3 as well.

Symptoms as well as TSH levels are used to monitor effect of substitution; a high TSH level suggests treatment is not yet adequate and that dose adjustments are necessary. A low ("suppressed") TSH may indicate the dose is too high. Some patients prefer a pig thyroid extract, natural desiccated thyroid hormones, which contains T3 as well as T4 and traces of T2, T1 and calcitonin.

References

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See also

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• hyperthyroidism
• thyroxine

External links

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• DMOZ
• Hypothyroidism Primer
• Network for Sustained Elimination of Iodine Deficiency

Thyroid disease

Хипотиреоидизъм | Hypothyreose | Hipotiroidismo | Kilpirauhasen vajaatoiminta | Myxœdème | 甲状腺機能低下症 | Hypothyreos