Hypothyroidism means hyposecretion of thyroid hormones ( ${\mathrm{T}}_3$ and ${\mathrm{T}}_4$ ). Thus levels of TSH increases. Since thyroid hormones are less metabolism does not take place and as a result patient experiences weight gain. This also causes bradycardia i.e. decreases heartbeat. Heat intolerance is the symptom of hyperthyroidism.