Although it is known that serum TSH concentrations remain unaffected in Ctsk−/− mice [14] and are significantly [10] or moderately elevated in Mct8 deficiency [11,15,16] in young or adult mice, respectively, no information is available on the outcomes of TSH receptor signaling in genotypes where Mct8 is lacking (i.e., Mct8−/y) or upregulated (i.e., Ctsk−/−). The gene discussed is SLC16A2; the disease is hyperinsulinemic hypoglycemia, familial, 4.