Thyroid function and effects on reproduction in ewes exposed to the organochlorine pesticides lindane or pentachlorophenol (PCP) from conception

There is concern over the potential endocrine-modulating effects of long-te rm exposure to pesticides. In this study, ewe lambs were exposed to lindane and pentachlorophenol (PCP) from conception to necropsy at 67 wk of age. T he ewe lambs land their mothers) were given untreated feed (n = 6) or feed treated with 1 mg/kg body weight/day of lindane (n = 8) or PCP (n = 13). Es trus war synchronized at 32 wk of age, and ewe lambs were exposed to vasect omized rams. Ewe lambs were then exposed to intact rams during the followin g two natural estrous periods and subsequent reproductive performance was m onitored. Serum was collected every 2 wk during development, daily during t he synchronized cycle and frequently (every 15-60 min) for 6-18 h either wi th or without stimulation with thyroid-stimulating hormone (TSH) during the synchronized luteal phase or TSH/thyroid-releasing hormone (TRH) at 65-66 wk of age. Ewe lambs fed a PCP-treated diet had a significantly reduced ser um concentration of both T-4 and free T-4, and a reduction in the magnitude and duration of the T-4 and free T-4 response to TSH, despite normal endog enous levels of TSH and a normal TSH response to TRH. PCP exposure had a le ss detrimental influence on unstimulated T-3 levels; however, the T-3 (but not reverse T-3) response to TSH was markedly reduced in Pc-P-treated ewe l ambs. Ewe lambs given lindane also had a significantly reduced serum concen tration of T-4; however, despite continued exposure to lindane, T-4 levels returned to normal by 10 wk of age. Detrimental effects on reproductive fun ction were only seen following estrous synchronization when both PCP and li ndane exposure reduced the number of corpora lutea (CL) and total CL volume and increased luteinizing hormone (LH) pulse frequency. In addition, linda ne-treated ewes had shorter estrous cycles and lower luteal progesterone co ncentrations. No marked effects of pesticides were seen on fertility follow ing mating during natural estrous periods. In conclusion, the pesticides af fected reproduction only after estrous synchronization, whereas PCP consist ently disrupted thyroid function, most likely through a direct effect on th e thyroid gland.