Let's get on to case number three. So this is Brody. Brody is an 11 year old male, neutered Labrador retriever mix. He has been previously healthy and is up to date on his vaccines. And he's presenting to you today for a routine senior health screening. As you're talking to his owner about how things have been going on, what's been going on, they note that he has had a decreased energy and activity level for about four months. They also note that he's been gaining weight, but they haven't been feeding him more and his appetite hasn't significantly changed. So here are his physical exam findings. And, you know, just to note, you know, his is heart rate is kind of towards the lower end of the reference range. He is obese. His body condition scores eight out of nine. On ocular exam he just has some lenticular sclerosis and iris atrophy, which is very common in older dogs. And then the other thing that you notice is that his hair coat is diffusely thinned and most pronounced on his lateral flanks bilaterally. 

So let's start by running some bloodwork. Let's start with our CBC this time. So here is our CBC and I'll just give you a second to review that. And here are our serum chemistries. And just like we talked about, if you note at the bottom here, my my pointer's gone away for a second, our T4 is right at the bottom here because we ran a senior panel and it's at 0.5. So if we look at our CBC will find that he has, and now this is something if you want to know more about this, go back to lecture one, he has a normocytic, normochromic, non-regenerative anemia. On his serum chemistries, he has a significant hypercholesterolemia, a mild increase in his ALP and his total T4 is decreased. 

Now we're going to look again at our thyroid hormone axis. So what we've got to remember, we're into the thyroid hormone now. We're getting deep into thyroid hormone. But a decrease in your total T4 does not equal hypothyroid, hypothyroidism. Okay? It is a very sensitive test. So the sensitivity, depending on the study that you read, is anywhere from 89 to 100%, but it's not specific. So what that means is you're going to get false positives. So if you just look at decreased total T4 alone, you are going to overdiagnose hypothyroidism. If your total T4 is in the mid to upper half of the reference range, hypothyroidism is ruled out. Okay? So you can clearly rule out hypothyroidism. If you have a decrease in your total T4, though, there are a variety of different reasons why that may be. And so true hypothyroidism may certainly be the case. But what you've got to remember is that other disease will affect your total thyroid hormone level. And so we call that sick euthyroid. So really any disease that is making an animal sick could cause you to have a decrease in your total T4. There are certain medications that also have a significant effect on your T4. So phenobarbital is one of those, sulfonamides, although they can actually cause a true hypothyroidism, glucocorticoids really importantly will change your thyroid metabolism and really any medication that a pet is receiving could potentially decrease your total thyroid hormone level. And then the normal range has been found to be decreased in some breeds. And so the big breed for that or the big breed group for that sight hounds. 

So we're going to talk about our free T4 again and you're going to do your free T4 by equilibrium dialysis because that's going to help us differentiate a true hypothyroid from a sick euthyroid. Okay. So it's the most accurate single test for hypothyroidism because like we already mentioned with cats free T4 is less commonly affected by non thyroidal disease. It has a 95% accuracy for diagnosing hypothyroidism. And just remember, we already mentioned this, but I'm just going to mention it again, the methodology or the way that you check your free T4 matters equilibrium dialysis is the most accurate method, and that's the method you should be using. So if you have let's look at two scenarios down here. Okay? We're going to look at two situations where we have a decrease in our total T4, but the normal free T4 sorry, free T4 is normal. That's going to mean that most likely what we're dealing with is a non thyroidal disease. Okay? Now, conversely, in this situation we have a decrease in our total T4, a decrease in our free T4, and we have a clinical suspicion of hypothyroidism. So in and if you remember in this dog, we had changes in the haircoat, we had lethargy, we had weight gain and then also we had other changes on serum chemistries and CBC that we'll get to in a second that strongly suggests that this pet may have hypothyroidism. 

Now, what about TSH? So we can potentially look at TSH in dogs with hypothyroidism as well. So going back to this negative feedback loop, remember that as your T4 decreases, that's going to stimulate increased production of TRH from your hypothalamus. That's going to stimulate increased TSH production from your pituitary to try to increase this T4. So we would expect in hypothyroidism that our TSH would be increased, but we're always going to look at that. We're always going to look at our TSH in relationship to total T4 and our free T4. So if we're going to look at TSH, we're going to look at those three things together. So if we have an increase in our TSH, with a decrease in our total T4 and a decrease in our free T4, we can feel fairly confident in saying that pet has hypothyroidism. If our TSH is normal, but our total T4 and our free T4 is low, but we have a clinical suspicion of hypothyroidism, it's important to remember that up to 25% of dogs who are hypothyroid have a normal TSH. okay? So this normal TSH does not rule out hypothyroidism. So we may still be hypothyroid in this scenario. If we have a normal TSH, with a decreased total T4 and a decreased or normal free T4 and no clinical signs, it's most likely that this is a non thyroidal illness. Okay? And so yet again, like we've already talked about in diabetes and in hyperthyroidism, we're always going to interpret thyroid hormone results in light of our clinical picture. So many factors, factors can influence total T4. And what's really important is that if you just look at total T4 to diagnose hypothyroidism, you will overdiagnosed this condition. 

All right. So there are some more advanced tests that we can do. And I'm not going to spend very long talking about these because it's kind of beyond, you know, really what you need to know for NAVLE®, but just so that you're aware, you can also measure a T3, T4 or thyroglobulin antibody level. And this is because the primary cause of hypothyroidism dogs is a lymphocytic inflammation of the thyroid. And so we might expect that these values would be increased. Now, very rarely a T4 auto antibody can cross react with our T4 assay. And so what happens in that case is that it will falsely increase our total T4 and our free T4 unless it's measured by equilibrium dialysis. If our free T4 is measured by equilibrium dialysis, it's not affected. Yet another reason why ED is the way to go. This is a very uncommon scenario, but it's just worth kind of putting out there. T3 really isn't looked at at this point. If you send a thyroid panel to Michigan State, which is sort of the the main thyroid testing lab in the US, they will include a T3, but we typically don't look at that. So auto antibodies are there, you can be aware of them, but it's pretty uncommon. As we already mentioned, primary hypothyroidism is very rare in adult cats and most commonly we see it due to I131 therapy for hyperthyroidism. 

All right, so what else might we see on our clinical pathology that makes us think that a patient may be hypothyroid? So I always look at my CBC for anemia. Okay? So in dogs that are truly hypothyroid, up to 44% of cases have a mild normocytic, normochomic, non-regenerative anemia. Hypercholesterolemia is very common. About three quarters of cases of hypothyroidism will have hypercholesterolemia and they may also have a fasting hyperlipidemia or hypertriglyceridemia. You may see a mild increase in creatinine kinase, and that's due to muscle damage, and that's in about 35% of cases. So if we have a decrease in our total T4, we have these clinical pathology changes and we have a clinical suspicion of hypothyroidism, that might be all we need to do to say this dog has hypothyroidism. If we want to kind of shore up that diagnosis, we can also run a free T4. And if that free T4 is low, like in this case, the free T4 is down at seven, that that picture together is very consistent with hypothyroidism. 

Okay, So I know it's a lot. So I've tried to kind of put it all down succinctly into a into a table or into a flowchart, I should say. So if you have a clinical suspicion of hypothyroidism and your total T4 is normal, hypothyroidism is really unlikely. Okay? So you can pretty much rule out hypothyroidism. If your total T4 is decreased. So it's below the reference range, which typically is less than one, we're going to perform our free T4 by ED or we're going to look at our other clin path changes. If we run our free T4 by ED and it's normal, then we can feel fairly confident in saying that hypothyroidism is unlikely. Instead, we're going to look at that patient for concurrent disease. If our free T4 by ED is decreased, that puts hypothyroidism much higher on our list of things that could be going on. We may want to run a TSH if we're really still questioning what's going on and we're really not sure about our diagnosis. But remember that in up to 25% of cases, the TSH may be normal. And so it doesn't always help us. So, you know, the key takeaway with these cases is that we always have to like look at our thyroid hormone level in light of our clinical picture. Okay. So you've always got to go back to what is my patient doing at home and what do the what does the other information tell me? Okay.