Effects of oral administration of
l-thyroxine (T
4) on growth performance, body composition, and some aspects of nutritional physiology were investigated in two slow-growing
air-breathing fish (
Channa punctatus and
Heteropneustes fossilis) under laboratory conditions (LD 12:12 at 25°C). The results indicate that irrespective of the species, fish fed diets containing
lower doses of T
4 (up to 50 mg kg
−1 of diet in
C. punctatus and up to 100 mg kg
−1 of diet in
H. fossilis) showed significantly (
P < 0.05) higher growth (in terms of live weight and length gain, specific growth rate, percentage gain in body weight and
condition factor), low feed conversion ratio, high nutrient retention, high apparent protein digestibility, and high digestive
enzyme activity. Viscero-somatic (VSI) and hepato-somatic (HSI) values were also high in fish fed on low dietary T
4 levels. Liver glycogen levels decreased with the increase in the dietary T
4 levels, indicating its utilization during growth. Muscle glycogen levels in
H. fossilis coincided with high growth at 100 mg of T
4. Observation of the postprandial excretion of metabolites (N–NH
4
+ and
o-PO
4
−) indicated significantly (
P < 0.05) low levels in aquaria water where the fish were fed diets with low T
4 levels. These studies further revealed that feeding fishes on higher T
4 levels (above 50 mg in
C. punctatus and above 100 mg in
H. fossilis) not only repressed growth performance and nutrient retention, but also affected carcass composition by lowering protein
accumulation (muscle and carcass protein) and energy assimilation. These studies revealed a biphasic action of thyroxine,
i.e., the hormone at lower doses is anabolic, while at higher doses it acts as a catabolic agent, indicating that feeding
fishes on higher doses can be detrimental to their growth and metabolism. In summary, the results of the present study show
that feeding
H. fossilis and
C. punctatus on low doses of T
4 enhances growth, decreases excretion of metabolites, and increases nitrogen retention. These observations suggest that T
4 supplementation of the diet may have practical utility in the culture of slow-growing fish species.
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