Thyroid hormones act on the kidney, liver, heart, nervous system, skeletal muscle and other tissues, making them more responsive to epinephrine and stimulating cellular respiration. Thyroid hormones act slowly and make take up to 48 hours after release to have any noticeable effect.

Functions of Thyroid Horomones

  • thermoregulation - because thyroid hormones increase the rate of cellular respiration, a related effect is the generation of heat. As a result, many vertebrates use this gland to control thermoregulation (maintaining body temperature).
  • salinity - in fish, the thyroid gland helps the organism adapt to the salinity of its environment. Secretions of the thyroid often specify the preference of a fish for salt or fresh water.
  • growth and development - hypothyroidism, low levels of thyroid hormone production can result in underdeveloped body structures and very low metabolism. This disease is known as cretinism in humans.
  • metamorphosis - the level of thyroid hormone production often correlates with the stage of metamorphosis.

thyroid hormone synthesis

Two major thyroid hormones are thyroxine and 3,5,3-triiodothryonine (also known as T4 and T3). They are synthesized by the iodination of two tyrosine amino acids. Iodine is acquired from the blood and sequestered in the thyroid gland.

Thyroid Stimulating Hormone (TSH) is synthesized in the adenohypophysis region of the brain and is responsible for stimulating thyroid hormone production in the thyroid gland. TSH is in turn upstream regulated by TSH Releasing Hormone (TRH) which is secreted by the hypothalamus and acts on the adenohypophysis. Negative feedback from thyroid hormones in the blood downregulates TRH production resulting in some degree of automatic maintenance of thyroid hormone levels in the blood. On top of this chemical regulation, the hypothalamus is able to control the amount of thyroid hormone in the body. Low skin temperature, for example, will trigger the hypothalamus to release TRH and thereby increase body temperature.

Thyroid hormones (levothyroxine, liothyroxine, liotrix, thyroid USP)

increase metabolic rates, resulting in increased cardiac output, oxygen consumption, body temperature, blood volume, growth, cellular development, respiratory rate, enzyme activity
Brand Names
Cytomel, Eltroxin, Levothroid tablets, Levothyroxine, Levothyroxine injection, Levoxyl, Synthroid, Synthroid injection, Tapazole, Unithroid
directly replaces thyroid hormone
adrenal insufficiency, myocardial infarction, thyrotoxicosis
elderly, angina pectoris, hypertension, ischemia, cardiac disease, diabetes mellitus, diabetes insipidus, pregnancy, lactation
Impaired thyroid absorption: cholestyramine
Increased effects: anticoagulants, sympathomimetics, tricyclic antidepressants, catecholamines
Decreased effects: digitalis, glycosides, insulin, hypoglycemics
Decreased thyroid effect: estrogens
Adverse Reactions
insomnia, tremors, tachycardia, palpitations, angina, dysrhythmias, weight loss, appetite changes, thyroid storm
Additional Information
Assess vitals, intake and output, weight daily, height and growth rate (child)
Assess thyroid function tests
Assess mental status, cardiac status
Administer at same time daily to maintain hormone level
Evaluate therapeutic response: absence of depression, increased weight loss, diuresis, pulse, appetite; absence of constipation, peripheral edema, cold intolerance, pale, cool, dry skin, brittle nails, alopecia, coarse hair, menorrhagia, night blindness, paresthesia, syncope, stupor, coma
Date of most recent Update
August 03, 2002
Further information is available in the writeup for the brand name(s) of this medication

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