Thyroid Function Recovery After Acute Illness

CHF Grant 02686-A: Pattern of Thyroid Function Tests during Recovery from Acute Nonthyroidal Illness

Publication:
Bolton TA, Panciera DL, Voudren CD, Crawford-Jennings MI. Thyroid function tests during nonthyroidal illness syndrome and recovery in acutely ill dogs. J Vet Intern Med. 2024; 38(1): 111-122. doi:10.1111/jvim.16947

What can we learn?
Numerous illnesses can alter hypothalamic-pituitary-thyroid axis function even though they do not directly affect the thyroid gland (known as euthyroid sick syndrome or nonthyroidal illness syndrome). Therefore, measuring thyroid hormones (TT4, fT4, or both) at a single point in time during acute illness may result in the inappropriate diagnosis of hypothyroidism and unnecessary hormone supplementation. Investigators prospectively measured thyroid hormones in 25 dogs with a low TT4 at admission hospitalized for nonthyroidal illness to assess the timeline of hormone level recovery.

Low TT4 levels in these dogs recovered rapidly, returning to normal in 80% of cases by discharge and 100% of cases by 2 weeks post-discharge. Free T4 (fT4) normalized within 2 days of admission, indicating that results of this assay are less affected by nonthyroidal illness and recover even faster than TT4.

Conclusion:
For an accurate diagnosis of hypothyroidism, clinicians should hold off on measuring thyroid function for 2-4 weeks after an acute illness. If thyroid function must be assessed immediately, free T4 by equilibrium dialysis (fT4ED) is the most accurate test option.