Effects of induced hypothyroidism or hyperthyroidism on growth and reproductive performance of Brahman heifers

Prepubertal Brahman heifers (BW = 302 +/- 7.5 kg, body condition score [BCS ] = 5.4 +/- .2, age = 498 +/- 3.4 d: SEM) were used to study the effects of thyroid function on growth and reproduction. Seven heifers were controls ( C). Seven heifers were induced to become hypothyroid by ingestion of 4 mg/k g BW of 6-n-propyl-2-thiouracil (PTU). Seven heifers were induced to become hyperthyroid (T) by daily s.c. injections of triiodothyronine (T-3, 1 mg/d ). Treatments were administered for 84 d followed by an 84 d posttreatment period. Blood samples were obtained twice weekly via tail venipuncture for analysis of T-3, thyroxine, and progesterone. The BW, BCS, and rectal tempe rature (RT) were recorded weekly. Estrus was monitored twice daily with the aid of a fertile bull equipped with a chin ball marker. Hyperthyroidism an d hypothyroidism were successfully induced in T- and PTU- treated heifers, respectively. During the treatment period, PTU heifers gained the most BW a nd BCS (72.4 +/- 5.4 kg; .93 +/- .15 units), C heifers were intermediate (4 1.7 +/- 5.4 kg; .43 +/- .15 units), and T heifers gained the least (13.3 +/ - 5.4 kg; -.36 +/- .15 units; P < .05). The RT also decreased (P < .05) in PTU heifers (-1.9 +/- .2 degrees C) compared with C (-1.2 +/- .2 degrees C) or T heifers (-.8 +/- .2 degrees C). No heifers exhibited estrus during th e treatment period. During the posttreatment period, T heifers gained the m ost BW and BCS (93.9 +/- 6.1 kg; 1.14 +/- .13 units), C heifers were interm ediate (67.0 +/- 6.1 kg; .86 +/- .13 units), and PTU heifers gained the lea st (22.2 +/- 6.1 kg; -.14 +/- .13 units; P < .05). The reversal in BW and B CS gains during the posttreatment period corresponded to periods of transie nt hypo- and hyperthyroidism in T and PTU heifers, respectively. Age and BW at puberty and pregnancy were similar among all treatment groups. The BCS for T heifers was lower (5.7 +/- .2 units; P < .05) at puberty and pregnanc y than for PTU heifers (6.6 +/- .2 units). Induction of hypothyroidism resu lted in significant increases in BW and BCS during the treatment period, bu t these increases were not sufficient to dramatically affect reproductive p erformance of Brahman heifers.