Hypothyroidism
Description
- The thyroid is a gland located slightly below the larynx in front of the trachea.
The three primary hormones produced by the thyroid gland are thryoxine (T4),
triiodothyronine (T3), and calcitonin. Thryoxine (T4) and
triiodothyronine (T3) maintain the level of metabolism in the tissues
that is optimal for their normal function. Calcitonin regulates the calcium and
phosphate levels in the blood.
- Hypothyroidism is characterized by undersecretion of thyroid hormones, resulting
in a lower basal metabolism.1
Causes
- Thyroid gland surgery, irradiation therapy, chronic autoimmune thyroiditis (Hashimotos
disease), or inflammatory conditions, such as amyloidosis and sarcoidosis.2
- Endemic goiter is the enlargement of the thyroid, usually due to inadequate amounts
of dietary iodine. This is particularly prevalent in the Midwest ("Goiter-Belt")
where low levels of iodine are found in the soil. However, in Japan goiter has
been associated with too much dietary iodine intake.3 For this reason
it is important to talk with your health care professional about the need for or
avoidance of iodine.
- Juvenile onset hypothyroidism may be caused by a congenital defect. Defects in
the thyroxine (T4) hormone are another cause.
Types
- Adult hypothyroidism has two classifications, primary and secondary. Primary
hypothyroidism comes from a disorder of the thyroid itself. Secondary hypothyroidism
is caused by an inability to stimulate the thyroid.
- Both types of hypothyroidism can lead to very severe complications including
myxedema coma.
At Risk
- People who consume inadequate amounts of iodine. Iodized salt has greatly reduced
this risk.
Prevention and Management
Nutritional Influences:
- Iodine is necessary for the formation of the thyroid hormones. Inadequate dietary
amounts of iodine may lead to goiter. Iodine can be found in seaweed and iodized
salt.
- Zinc and selenium deficiencies affect the metabolism of thyroid hormones.4
- Selenium is important for modulating T4 levels.5
Additional Information
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Abstracts
Als C, Lauber K, Brander L, Luscher D, Rosler H. The instability of dietary
iodine supply over time in an affluent society. Experientia 1995 Jun 14;51(6):623-33. In
the Bernese region, where goiter was formerly endemic, alimentary salt has been
supplemented by increasing amounts of potassium iodide (KI): 5, 10, 20 mg KI/kg
in 1922, 1965 and 1980 respectively. Ioduria rose from < 30 micrograms I/g
creatinine in 1920 to > 100 micrograms I/g creatinine in the 1980s. In 1992
ioduria was estimated in 55 healthy volunteers (group A and individual B) and
234 thyroid carcinoma patients after thyroidectomy: hypothyroid patients with
(C) and without thyroid remnants (D) and euthyroid patients on T4 substitution
(E). The arithmetic mean iodine excretion of the healthy volunteers in group
A and individual B was found to be 87 +/- 40 micrograms I/g creatinine. This
is insufficient according to the recommendations of the WHO. In all groups, the
iodine excretion reached the recommended level only in some members: 24% (A,
B), 19% (C), 38% (D) and 81% (E). It was thought in the 1980s that in a formerly
iodine-deficient society, iodinated salt would continue to provide an adequate
supply of iodine. However, iodine intake in this affluent society has proved
to be unstable. This can be attributed to modifications of eating habits, which
include a reduction of total salt consumption, combined with a growing consumption
of manufactured food of cosmopolitan origin, prepared using salt containing little
or no iodine.
