Normal human stem cells rely on low levels of active telomerase to sustain their high replicative requirements. Deficiency in telomere maintenance mechanisms leads to the development of premature aging diseases, such as dyskeratosis congenita (DC) and aplastic anemia (AA). Mutations in the unique insertion in fingers domain (IFD) in the human telomerase catalytic subunit (hTERT) have previously been identified and shown to be associated with DC and AA. However, little is known about the molecular mechanisms impacted by these IFD mutations. We performed comparative functional analysis analyses of disease-associated IFD variants at the molecular and cellular level. We report that hTERT-P721R- and hTERT-R811C-expressing cells exhibited growth defects likely due to impaired TPP1-mediated recruitment of these variant enzymes to telomeres. We showed that activity and processivity of hTERT-T726M failed to be stimulated by TPP1-POT1 overexpression and that dGTP usage by this variant is was less efficient compared to the wild-type enzyme. hTERT-P785L-expressing cells did not show growth defects and this variant likely confers cell survival through increased DNA synthesis and robust activity stimulation by TPP1-POT1. Altogether, our data suggest that multiple mechanisms contribute to cell growth defects conferred by the IFD variants.