Atypical Thyroid Function Tests and Thyroid Hormone Resistance

Abstract

Clinical interpretation of thyroid labs is usually straightforward. However, there are rare instances when the atypical profile of thyroid labs warrants systematic investigation to determine the underlying cause. We report the case of a 90-year-old Caucasian male with a chronic history of atrial fibrillation with chronic pacemaker dependence who presented with significantly elevated free thyroxine level (>7.77 ng/dL) but normal thyroid-stimulating hormone level (2.15 µIU/mL). After ruling out pituitary tumors and artifactual errors due to lab interference, the diagnosis of thyroid hormone resistance was made.

Diagnosing a Probable Case of Thyroid Hormone Resistance in a Resource-Poor Setting

“…This may mean that with treatment, there may be some intrinsic recovery from thyroid hormone resistance as reported by some authors. (Rivas et al, 2016;Lee et al, 2022;Singh et al, 2017;Pattan et al, 2021) Therefore, monitoring of the index patient will continue with the hope that intrinsic recovery from the thyroid hormone resistance continues, possibly manifesting with the appearance of toxic symptoms again, that will necessitate a further reduction in the daily dose of levothyroxine. The patient has, however, remained clinically stable at a daily 400 µg levothyroxine dose, even with elevated thyroid hormones and unsuppressed TSH.…”

“…(Singh et al 2017;Moran et al 2017) It can present as asymptomatic, hypothyroidism, hyperthyroidism, or a combination of symptoms of hypothyroidism and hyperthyroidism depending on the level of expression of THRB and THRA genes in target tissues, (Neamţu et al, 2016) thus very confusing to clinicians not familiar with the condition, leading to misdiagnosis, unnecessary invasive treatments like radioactive iodine ablation due to refractory goiters, and frustrating to patients. (Pattan et al, 2021) Two forms of RTH have been described i.e., Pituitary Resistance to Thyroid Hormone (PRTH) which presents predominantly with clinical hyperthyroidism; and Generalized Resistance to Thyroid Hormone (GRTH) which tends to present with clinical hypothyroidism. (Neamţu et al, 2016) The symptoms tend to wane with age, and patients may eventually become clinically euthyroid.…”