The LDIFLARE and CCM Methods Demonstrate Early Nerve Fiber Abnormalities in Untreated Hypothyroidism: A Prospective Study

Context Recent studies using skin biopsy suggest presence of small-fiber neuropathy in subclinical hypothyroidism. This study uses two noninvasive methods—the laser Doppler imager flare technique (LDIFLARE) and corneal confocal microscopy (CCM)—to assess small-fiber function (SFF) and small-fiber structure (SFS), respectively, in newly diagnosed hypothyroidism (HT) before and after adequate treatment. Design and Setting Single-center, prospective, intervention-based cohort study. Patients and Participants Twenty patients with newly diagnosed HT (15 with primary HT and 5 with post-radioiodine HT) along with 20 age-matched healthy controls (HCs). Interventions Patients with HT and HCs were assessed neurologically at diagnosis and baseline, respectively. The HT group was reassessed after optimal replacement (defined as TSH level of 0.27 to 4.20 mIU/L) with levothyroxine (LT4) and HCs were reviewed after 1 year. Main Outcome Measures Neurologic assessment for small fibers was performed by using LDIFLARE for SFF and CCM for SFS; large fibers were studied by sural nerve conduction velocity (SNCV) and sural nerve amplitude (SNAP). Results At baseline, both LDIFLARE (mean ± SD) (6.74 ± 1.20 vs 8.90 ± 1.75 cm2; P = 0.0002) and CCM nerve fiber density (CNFD) (expressed as number of fibers per mm2: 50.77 ± 6.54 vs 58.32 ± 6.54; P = 0.002) were significantly reduced in the HT group compared with HCs whereas neither SNCV nor SNAP was different (P ≥ 0.05). After optimal LT4 treatment, both LDIFLARE (7.72 ± 1.12 vs 6.74 ± 1.20 cm2; P ≤ 0.0001) and CNFD (54.43 ± 5.70 vs 50.77 ± 6.54 no./mm2; P = 0.02) improved significantly but remained significantly reduced compared to HCs (P = 0.008 and P = 0.01, respectively) despite normalization of TSH. Conclusions This study demonstrates that dysfunction of small fibers precedes large neural fiber abnormalities in early HT. This can be reversed by replacement therapy to achieve a biochemically euthyroid state, but small-fiber neural outcomes continued to remain low compared with values in HCs.