BACKGROUND
Hypothyroidism is a common disorder affecting the general population and thyroid hormone is one of the most frequently prescribed medications in the United States. Current guidelines recommend the use of levothyroxine as the recommended form of thyroid hormone replacement. However, some patients and practitioners prefer desiccated thyroid extract (DTE) to treat hypothyroidism as a more “natural” option for treatment. DTE is a thyroid hormone pill made from animal thyroid glands. DTE has a significantly higher content of T3 than human thyroids produce, which may lead to blood levels higher than normal of this hormone. Also, T3 has a short half-life which causes daily fluctuations in T3 levels in patients taking these preparations. In addition, there have been concerns regarding the consistency of the amount of hormones batch to batch in DTE products.

Some small studies have reported some patients experience better symptom control and quality of life when taking DTE compared with levothyroxine. However, there are no long term randomized clinical studies that evaluate the effects of DTE on bone health or risk for atrial fibrillation, which are adverse effects if the thyroid hormone replacement dose is too high.

Thyroid stimulating hormone (TSH) provides information about thyroid hormone levels and is used to monitor thyroid hormone therapy and assess whether a dose is adequate. This study was done to understand whether the level of TSH varies significantly over time in patients being treated with DTE as compared to TSH levels in patients taking levothyroxine.

THE FULL ARTICLE TITLE
Kuye R et al 2020 Thyroid Stimulating Hormone Stability in Patients Prescribed Synthetic or Desiccated Thyroid Products: A Retrospective Study. Ann Fam Med 18:452–454.

SUMMARY OF THE STUDY
This study was done using data from electronic medical records, laboratory and pharmacy records from Kaiser Permanente Colorado patients. Patients who were at least 18 years of age, who had been treated with either levothyroxine or DTE between 2005 and 2015 were eligible to be included. Patients with diagnosis of Hashimoto’s thyroiditis, pituitary disease, thyroid cancer, pregnancy, treatment with radioactive iodine were excluded. Data was collected for a 3 year period since the first date a patient filled a prescription. The main outcome examined was the proportion of TSH values within the normal range (0.32-5.5 mIU/L). Other outcomes examined included the proportion of TSH values in the normal range throughout the 3 year period, the variability of TSH values between visits and the number of TSH values.

There were a total of 435 DTE users and 435 levothyroxine users. The TSH was checked more frequently in the levothyroxine users than in the DTE users, but the difference was not significant. The proportion of TSH in normal range did not differ between the groups (levothyroxine users: 79.1%; DTE users: 79.3%). Although 60% of patients in each group had TSH values that stayed in the normal range throughout the 3 years, there was more visit-to-visit variability in the TSH values of the DTE users than levothyroxine users.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study suggests that the proportion of the TSH values within reference range did not differ between patients using DTE or levothyroxine for treatment of hypothyroidism. A strength of this study is the fact that they matched patients 1:1 on sex, age and race/ethnicity for both groups. However, the actual reason for the hypothyroidism was not described, as individuals with the diagnosis of Hashimoto’s thyroiditis and other common causes for hypothyroidism were excluded. It is likely that patients who had hypothyroidism due to surgery represented most of patients studied and therefore it is not clear whether the results would have been the same had the other causes been included. However, the fact that this study showed less variability visit to visit in TSH levels in patients who were taking levothyroxine suggests that levothyroxine remains the preferable treatment for patients for whom very little TSH variability is desired such as those with thyroid cancer or women during pregnancy.

— Jessie Block-Galarza, MD

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