Clinical Thyroidology® for the Public

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Use of levothyroxine absorption tests

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Hypothyroidism is a disorder in which the body does not have enough T4, an essential hormone. The defect arises from either the thyroid or less commonly, from the pituitary. The most common thyroid disorder in the U.S. is called Hashimoto’s thyroiditis, in which the body gets confused and makes antibodies that will destroy the thyroid gland, causing hypothyroidism.

To supply the body needs, most patients take oral levothyroxine, which is the T4 hormone, on a daily basis. The dose of levothyroxine needed to return the low levels seen in hypothyroidism back to normal can be variable and affected by a number of factors, including a patient’s weight, other medications that the patient may be taking and whether the patient misses any doses. Missing doses is a common cause of erratic thyroid hormone levels in the blood in hypothyroid patients but many other factors can also cause difficulty in controlling hypothyroidism and returning thyroid hormone levels back to normal. In general, levothyroxine is absorbed well in the intestine. However, levothyroxine’s absorption by the intestines can decrease when patients take it with other medications, or when taken with food or vitamins, or in cases of celiac disease or other diseases. This can result in high doses of levothyroxine being required to get thyroid hormone levels back into the normal range and also lead to erratic thyroid hormone levels despite taking levothyroxine regularly.

To evaluate whether there are problems absorbing levothyroxine in patients with hypothyroidism, several absorption tests have been developed and published. This study examine the use of these levothyroxine absorption tests in patients with hypothyroidism.

Caron P and Declèves X 2023 The use of levothyroxine absorption tests in clinical practice. J Clin Endocrinol Metab. Epub 2023 Mar 14. PMID: 36916146.

The study looked at 43 published studies. The authors reviewed procedures used, levothyroxine dose and formulation, duration of the test, and frequency of blood tests and other analysis. The tests varied in several topics, such as fasting/ non-fasting, levothyroxine doses and even if the patient was supervised. After reviewing multiple published variations of oral levothyroxine absorption tests, the authors made the following key recommendations:

  • The oral levothyroxine test dose should be greater than 300 μg; the authors suggest a 1000-μg dose.
  • The FT4 level should increase more than 0.40 ng/ dl, or a total T4 level should increase more than 6 μg/dl to a test dose to be considered normal absorption.
  • If the test indicates abnormal levothyroxine absorption, options include cautiously increasing the levothyroxine dose with monitoring, changing to a different brand or formulation of levothyroxine, and/or referring the patient to a gastroenterologist for evaluation of malabsorption.

Erratic thyroid hormone levels and high dose requirements for levothyroxine are often seen in patients with hypothyroidism. While missing doses is a common cause of erratic thyroid hormone levels, it is important to identify possible causes for decreased levothyroxine absorption, as patients with decreased absorptions of levothyroxine might also suffer from decreased absorption of vitamins and other important elements for the body. This study identified potential tests evaluating the absorption of levothyroxine and recommended a simple clinically applicable test.

— Joanna Miragaya, MD


Autoimmune thyroid disease: a group of disorders that are caused by antibodies that get confused and attack the thyroid. These antibodies can either turn on the thyroid (Graves’ disease, hyperthyroidism) or turn it off (Hashimoto’s thyroiditis, hypothyroidism).

Hashimotos thyroiditis: the most common cause of hypothyroidism in the United States. It is caused by antibodies that attack the thyroid and destroy it.

Hypothyroidism: a condition where the thyroid gland is underactive and doesn’t produce enough thyroid hormone. Treatment requires taking thyroid hormone pills.

Levothyroxine (T4): the major hormone produced by the thyroid gland and available in pill form as Synthroid™, Levoxyl™, Tyrosint™ and generic preparations.

Thyroid hormone therapy: patients with hypothyroidism are most often treated with Levothyroxine in order to return their thyroid hormone levels to normal. Replacement therapy means the goal is a TSH in the normal range and is the usual therapy. Suppressive therapy means that the goal is a TSH below the normal range and is used in thyroid cancer patients to prevent growth of any remaining cancer cells.

Thyroxine (T4): the major hormone produced by the thyroid gland. T4 gets converted to the active hormone T3 in various tissues in the body.

TSH: Thyroid Stimulating Hormone — produced by the pituitary gland that regulates thyroid function; also the best screening test to determine if the thyroid is functioning normally.