We started looking at it in PRRS and what we found is, in serious PRRS infections, whether it's in a fetus, a gilt or a nursery pig, when they reach that peak thyrenia they will be hypothyroid. Hypothyroidism isn't typically something you would associate with infections.
Generally, if something is hypothyroid that means they have an issue with their thyroid gland itself or in the hypothalamic pituitary thyroid access which controls the set point level of thyroid hormone inside the body. But in significant disease, so infections where the pigs are getting really quite sick, there's actually a suppression of that thyroid hormone and, when the animals reach their peak sickness, their thyroid hormones drop to the lowest they'll drop to.
In the case of PRRS we can see that the thyroid hormones drop to about 50 percent of normal levels. In other diseases that we've started looking at such as Porcine circovirus or Mycoplasma hyopneumoniae or Salmonella infection, it won't be to the same extent as we've seen in PRRS but certainly if you look at the animals that aren't growing and aren't performing as well, those animals are the ones that are having a transient hypothyroidism.
As they get sick, their thyroid hormone levels are starting to drop and those are the animals that aren't growing and aren't performing as well.
Source : Farmscape