In order to compare the effects of addition of fluorine (F) in diets differing in protein content on the urinary F excretion, blood profile and thyroid hormones, 30 crossbred calves (6-8 months) initially exposed to different protein levels were allotted into six groups in a 3 × 2 factorial design. The factors included three different levels of protein: normal (NP; 100%), low (LP; 75%), and high (HP; 125%) besides two levels of supplemental fluorine (as sodium fluoride) at 0 or 200 mg/kg diet. The animals were fed a wheat straw-based diet for 210 d. Feeding NP diets decreased urinary fluoride excretion, measured at 42 d intervals, compared with LP diets. Blood levels of hemoglobin and haematocrit, measured at 70 d intervals, were not affected by either protein or fluorine levels, but period differences were apparent. Serum levels of urea, alkaline phosphatase and F showed greater increase in groups supplemented with F than in those without it; however, serum calcium was higher in the latter. Serum tri-iodo-thyronine and thyroxine levels were higher in HP and NP fed calves than animals on LP diets, respectively. The animals fed LPF have higher urinary F as compared with animals fed NPF, but were not different from the group fed HPF. The blood and serum variables indicated that there is no extra protection against susceptibility to F toxicity upon feeding of 25 percent higher protein than requirements.
[N]o effect of F on thyroid hormones was noticed in the present study. This was contradictory to the findings of Wheeler & Feel (1983), who reported a significant fall in T3 and T4 levels because of adverse impact of fluorosis on the thyroid glands. It was reported that 250 mg/kg and 400 mg/kg F in diet significantly decrease serum T4 and FT4 levels and increase serum TSH, but no significant differences were observed in serum T3 and FT3 after 50 days of treatment (Zhan et al., 2006). Similarly, Hillman et al. (1979) found decreased level of circulating T3 and T4 levels in cows with severe lesions of F toxicity, whereas other researchers found non-significant decrease in T4 values in buffaloes fed F at 60 mg/kg for 90 days (Madan et al., 2009). From these observations it appears that a decrease in thyroid hormone ensues only in case of severe F toxicity, which perhaps, may not be the case in the present study.