Clinical Thyroidology® for the Public

Summaries for the Public from recent articles in Clinical Thyroidology
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HYPOTHYROIDISM
Increased FT4 and incompletely suppressed TSH levels are mainly related to levothyroxine use

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BACKGROUND
Thyroid stimulating hormone (TSH) is a pituitary hormone that binds to the thyroid gland, helping it grow and produce thyroid hormones. The main thyroid hormone is thyroxine and is often measured as free thyroxine (FT4). TSH levels are opposite to FT4 levels. When the thyroid is underactive and FT4 levels are low, TSH levels go up. When the thyroid is overactive and FT4 levels are high, TSH levels go down. Patients taking levothyroxine for underactive thyroid have their TSH levels monitored to make sure that their levothyroxine dose is not too much or too little. If the TSH is either low or high, FT4 levels are usually measured next.

The authors of this study noticed that when they measured thyroid levels in their hospital, in many patients there was an unexpected pattern: the patients had high FT4 levels even though their TSH levels were normal, not low, as it would be expected. Since they had just started using a new FT4 assay, this study was done to compare the FT4 levels with both the new assay and the old assay to determine what was the cause of these results.

THE FULL ARTICLE TITLE
 Jansen HI et al 2023 Increased fT4 concentrations in patients using levothyroxine without complete suppression of TSH. Endocr Connect 12(4):e220538. PMID: 36762702.

SUMMARY OF THE STUDY
The authors studied blood samples from 50 healthy volunteers. They used two different assays to measure the hormones. They also looked at results of TSH and FT4 in over 3000 patients and compared the results obtained with new and old assays. They also looked at those patients with high Ft4 with normal TSH levels to see if they were taking thyroid medication.

The results of the testing showed that the new normal ranges were consistent with the prior normal ranges and the old and new FT4 assays gave similar results. There were slightly more results with high FT4 and normal TSH levels with the new assay (4,4% vs 2.3%). Interestingly, up to about 80% of the results with high FT4 and normal TSH levels were seen in patients taking levothyroxine.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
These data show that 2-4% of individuals will have an increased FT4 and normal TSH and that ~80% of these results will be obtained in patients currently taking levothyroxine. Measuring just TSH for the monitoring of thyroid levels in patients taking levothyroxine may be more useful of the TSH level returns in the normal range. However, if the TSH returns abnormal (either high or low), a FT4 level can be helpful.

— JSusana Ebner MD

ABBREVIATIONS & DEFINITIONS

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.

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

Thyroid stimulating hormone (TSH): 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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