Without going deep here, Serum Test or Total test just tells you how much is floating about and gives no indication of how much is bound and available or bound and unavailable (Typical Test levels range from 300 to 1000 ng/dl, like I said mine was 330 which in all realism is within the normal range but still low).
Circulating testosterone is chiefly bound in the serum to sex hormone-binding globulin (SHBG) and albumin. The albumin-bound fraction of testosterone easily dissociates from albumin and is presumed to be bioactive. The portion of testosterone bound to SHBG is not considered biologically active. The amount of SHBG in the serum and the total testosterone level will determine the distribution of bioactive and nonbioactive androgen. Approximately 40% of testosterone in plasma is bound to SHBG, 2% remains unbound (free) and the rest is bound to albumin and other proteins, this is the basis for the Free Androgen Index.
Thyroid levels can play havoc with your mood and energy if they are too low.
Both or either of these can be causing your lethargy which is why I said have them do a full panel.