Effect of Combined Levothyroxine (L-T(4)) and 3-Iodothyronamine (T(1)AM) Supplementation on Memory and Adult Hippocampal Neurogenesis in a Mouse Model of Hypothyroidism

Mood alterations, anxiety, and cognitive impairments associated with adult-onset hypothyroidism often persist despite replacement treatment. In rodent models of hypothyroidism, replacement does not bring 3-iodothyronamine (T(1)AM) brain levels back to normal. T(1)AM is a thyroid hormone derivative with cognitive effects. Using a pharmacological hypothyroid mouse model, we investigated whether augmenting levothyroxine (L-T(4)) with T(1)AM improves behavioural correlates of depression, anxiety, and memory and has an effect on hippocampal neurogenesis. Hypothyroid mice showed impaired performance in the novel object recognition test as compared to euthyroid mice (discrimination index (DI): 0.02 ± 0.09 vs. 0.29 ± 0.06; t = 2.515, p = 0.02). L-T(4) and L-T(4)+T(1)AM rescued memory (DI: 0.27 ± 0.08 and 0.34 ± 0.08, respectively), while T(1)AM had no effect (DI: −0.01 ± 0.10). Hypothyroidism reduced the number of neuroprogenitors in hippocampal neurogenic niches by 20%. L-T(4) rescued the number of neuroprogenitors (mean diff = 106.9 ± 21.40, t = 4.99, p(corr) = 0.003), while L-T(4)+T(1)AM produced a 30.61% rebound relative to euthyroid state (mean diff = 141.6 ± 31.91, t = 4.44, p(corr) = 0.004). We performed qPCR analysis of 88 genes involved in neurotrophic signalling pathways and found an effect of treatment on the expression of Ngf, Kdr, Kit, L1cam, Ntf3, Mapk3, and Neurog2. Our data confirm that L-T(4) is necessary and sufficient for recovering memory and hippocampal neurogenesis deficits associated with hypothyroidism, while we found no evidence to support the role of non-canonical TH signalling.