PSI-3 - Induced hypothyroidism causes depressed feed intake and growth in nursery pigs

Abstract: Hypothyroidism has been connected to pathogenic stress in previous research however, the exact role that it plays in reducing body weight and appetite during infection is unknown. Nursery pigs experiencing a polymicrobial disease or Porcine Reproductive and Respiratory Syndrome Virus challenge have lowered triiodothyronine (T3) and thyroxine (T4) concentrations and decreased average daily gain (ADG). We hypothesized that hypothyroidism causes the decreased ADG following pathogenic stress by reducing appetite and altering basal metabolism, leading to decreased feed intake and altered growth patterns. To examine this cause-and-effect relationship, we utilized a previously established non-pathogenic model of post-natal hypothyroidism based on the antithyroid drug methimazole (MMI). A total of n=28, 5-week-old barrows were placed in metabolism crates for 30 days, with administration of 10 mg/kg/day MMI or equivalent sham control for 23 days following a 7-day acclimation period, in 2 repetitions of n=12 and n=16 pigs. Water usage and skin temperature were recorded daily, body weight, feed intake (ADFI) and rectal temperature twice weekly and serum collected weekly. On day 21 (D21) of MMI treatment, backfat was measured via ultrasound. Pigs were euthanized on D23 for intensive phenotyping including organ weights, loin area, and crown rump and intestine length. An increase in postmortem thyroid weight (P < 0.0001) in conjunction with goitrous histology indicates successful induction of central hypothyroidism. Beginning at D7, MMI pigs had decreased T4 concentrations (P < 0.001) while T3 concentrations remained similar to control pigs (P >0.05), indicating mild peripheral hypothyroidism. This is consistent with prior studies using similar MMI dosage to induce hypothyroidism in nursery pigs. MMI reduced ADFI and body weight from D14 onward (P < 0.05), with effects becoming more significant with time. ADG was reduced at D10 (P < 0.05), tended to be reduced at D14 (0.05< P< 0.10), and was reduced at D21 (P < 0.001), while feed efficiency was not altered at any time point. Therefore, the reduction in ADG and body weight are likely related to reduced ADFI. Control and MMI pigs did not differ in water usage, rectal or skin temperature, loin area, moisture content of liver, heart, and loin, or major organ weight (P >0.05), except for the hindgut, which was smaller in MMI pigs (P < 0.05). Backfat depth and crown rump length were reduced (P < 0.05) in MMI pigs compared to control. This indicates that the difference in body weight may be due to less fat deposition in MMI pigs as well as reduced length. Overall, MMI induced mild hypothyroidism in nursery pigs which led to decreased ADFI, and subsequently decreased body weight characterized by decreased backfat and shorter body length. This indicates that hypothyroidism may be a contributing factor in reduced weight gain due to pathogenic stress in swine.