TSH Normal, FT4 Trending Up: What Risk for Afib?

Is this patient, status post motor vehicle accident, at risk for developing atrial fibrillation?

You were rounding late on a Friday afternoon when you were paged to see a24-year-old patient with atrial fibrillation (AF) in the ICU. The arrhythmia was secondary to acute respiratory distress syndrome and appropriate management was initiated.

You thought you were on your way home for the weekend when the medical resident who paged you decides that you have been so helpful on this young patient that he wants to “curbside” you on another patient in the ICU.

The patient is a 48-year-old male who was brought to the ICU after a motor vehicle accident and was found to have a normal TSH but upper limits of normal FT4.

The resident asks if this patient is at increased risk for developing AF.

What do you answer-and why?

A. Yes

B.  No

Please click "next" below for answer and discussion.

Answer: A. Yes, the patient is at increased risk for developing AF.

A recent systematic analysis published in Circulation1 looked at quartiles of FT4 within the normal range in the setting of normal TSH or “subclinical hypothyroidism” (elevated TSH, nl FT4, which was 6.5% of the cohort) and risk for developing incident AF in 30,085 patients ≥40 years of age. Compared with the lowest FT4 quartile in the normal range, AF risk increased 17% in the second quartile (HR 1.17, 95% CI 1.02–1.35), 25% in the third quartile (HR 1.25, 95% CI 1.09–1.43), and 45% in the highest quartile (HR 1.45, 95% CI 1.26–1.66) after adjustment for age and sex. Baseline TSH was not associated with incident AF. These results are consistent with the result of the Rotterdam study2 that showed that there is a higher incidence of AF with higher FT4 levels within the normal range, especially in younger subjects, and this slightly improves risk prediction.

The explanation the authors offer is that FT4 may be the hormone that acts on the myocardium directly and therefore, the FT4 level may be more relevant to development of this cardiac arrhythmia.


References:1. Baumgartner C, da Costa BR, Collet TH, et al. Thyroid function within the normal range, subclinical hypothyroidism, and the risk of atrial fibrillation. Circulation 2017; DOI:10.1161/CIRCULATIONAHA.117.028753.

2. Chaker L, Heeringa J, Dehghan A, et al. Normal thyroid function and the risk of atrial fibrillation: The Rotterdam Study. J Clin Endocrinol Metab 2015;100:3718-2724.

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