Clinical Thyroidology® for the Public
Summaries for the Public from recent articles in Clinical Thyroidology
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THYROID HORMONE REPLACEMENT
Are different batches of levothyroxine consistently treating hypothyroidism?
BACKGROUND
Hypothyroidism, or underactivity of the thyroid gland, comes in 2 general forms: primary hypothyroidism (high TSH and low T4) and secondary hypothyroidism (low/ normal TSH, low T4 and T3). Primary hypothyroidism is most commonly caused by an autoimmune thyroid disease called Hashimoto’s thyroiditis which causes destruction of the thyroid gland. Primary hypothyroidism can also be caused by pregnancy, minor illnesses, lithium, surgery, and some medications like tyrosine kinase inhibitors used to treat thyroid and other cancers. Secondary hypothyroidism occurs when the pituitary gland in the head cannot send the thyroid gland in the neck enough TSH.
This study was done to see if TSH levels that are monitored during the treatment of both types of hypothyroidism are significantly changed when switching to/from generic or brand name formulations of levothyroxine hormone replacement therapy. This study is challenging because of weight, absorption, compliance, and medication interaction changes that might alter TSH values regardless of the medication itself. The goal of this study is to look at the frequency of normal TSH values and likelihood of abnormal TSH values when different levothyroxine preparations are taken.
THE FULL ARTICLE TITLE
Bianco AC et al Levothyroxine Treatment Adequacy and Formulation Changes in Patients With Hypothyroidism: A Retrospective Study of Real-World Data From the United States Thyroid, 2023, online.
SUMMARY OF THE STUDY
Patients diagnosed with either primary or secondary hypothyroidism between the years of 2013 and 2020 were randomly chosen from a medical database. Patients were chosen mostly from the clinic setting with at least two TSH values spaced at least 1-15 months apart were included, and those who switched levothyroxine formulations were identified.
Out of a total 11,259 patients, 81.1% of these patients had a normal TSH for their first measurement. Subsequently, 24.9% of these patients had at least one formulation change for their levothyroxine prescription. These patients were matched by age, sex, and insurance type and still greater than 70% had a normal TSH at the second (follow up) measurement in these matched categories. However, 11.7% of the patients had a moderately high TSH, meaning the levothyroxine dose was too low. In addition, 7.3% of the patients had a moderately low TSH, meaning the levothyroxine dose was too high. Only 4% of the patients had very low TSH values below 0.1mIU/L or very high TSH values above 10 mIU/L.
WHAT ARE THE IMPLICATIONS OF THIS STUDY?
Most hypothyroid patients (81.1%) have a normal TSH on their levothyroxine medication, despite 24.9% of these patients having at least one formulation change (whether in between generic or brand name). This suggests that a change from generic to brand levothyroxine or vice versa does not significantly change TSH levels. Importantly, this study did not include any clinical symptoms associated with the change in levothyroxine formulations, so that cannot be evaluated. However, this study is helpful to show that the majority of patients with a normal TSH on one levothyroxine formulation will have a normal TSH on another formulation.
— Pinar Smith, MD
ATA RESOURCES
Hashimoto’s Thyroiditis: https://www.thyroid.org/hashimotos-thyroiditis/
Hypothyroidism (Underactive): https://www.thyroid.org/hypothyroidism/
Thyroid Function Tests: https://www.thyroid.org/thyroid-function-tests/
Thyroid Hormone Treatment: https://www.thyroid.org/thyroid-hormone-treatment/
ABBREVIATIONS & DEFINITIONS
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).
Hypothyroidism: a condition where the thyroid gland is underactive and doesn’t produce enough thyroid hormone. Treatment requires taking thyroid hormone pills.
Primary hypothyroidism: the most common cause of hypothyroidism cause by failure of the thyroid grand.
Secondary hypothyroidism: a rare cause of hypothyroidism where the thyroid gland is normal and the problem is inadequate TSH secretion from the pituitary gland.
Hashimotos thyroiditis: the most common cause of hypothyroidism in the United States. It is caused by antibodies that attack the thyroid and destroy it.
Pituitary gland: this endocrine gland sits at the base of the brain and secretes hormones that control thyroid and adrenal function, growth and reproduction. The pituitary gland secretes TSH to control thyroid function.
Levothyroxine (T4): the major hormone produced by the thyroid gland and available in pill form as Synthroid™, Levoxyl™, Tyrosint™ and generic preparations.
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.
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