Euthyroid Sick Syndrome

You’re presented with a middle-aged patient with lethargy and weight gain, you decided to run blood work including a total T4, and the only thing on blood work is a lower than normal total T4.


The next step is determining if that low total T4 is truly hypothyroidism or if there is something else that is lowering that T4.


I have seen a lot of patients, I mean, relatively speaking, but many of the so called hypothyroid patients that were started on thyroid replacement therapy, without any further testing. Hypothyroidism is one of the most over-diagnosed diseases in dogs. Please, do not be that clinician.

If you think that supplementing a little thyroid medication in a “suspect” hypothyroid dog is not going to have any consequences, you have not had to manage and then diagnose an iatrogenic hyperthyroid patient!



There is no evidence that thyroid hormone supplementation is beneficial in dogs with sick euthyroid syndrome, and it may be detrimental.


Case: 4yo MN Doberman Pinscher

This case came to me during emergency hours on a holiday. The patient was being referred for a full abdominal ultrasound due to severe elevation of liver enzymes and an echocardiogram. The ultrasound was taking place after the holiday, but this patient wasn’t eating so naturally the owner was very concerned.


We pulled a blood sample and assessment hydration. Since the patient was dehydrated we admitted for hospitalization and monitoring. The patient was thought to have heart failure and put on furosemide at a high dose by its regular veterinarian. The patient was then clinically dehydrated, so I trickled in fluids and warned the owner of any concurrent heart failure being an issue, so it was going to be a balancing act.


The bloodwork came back with elevated blood urea nitrogen, liver enzymes (ALP and ALT) and when I found out through the history that the patient was ‘diagnosed’ with hypothyroidism, then about 6 months later had a cancerous tumour removed from its leg, I was suspicious of the diagnosis of hypothyroidism as being misdiagnosed and the patient was actually euthyroid sick with cancer.


So what is euthyroid sick syndrome?


Euthyroid itself means normal thyroid gland. Euthyroid sick is when you have a normal thyroid gland, but an abnormally low T4 due to some other illness. The pathophysiology of why thyroid hormones are decreased with chronic illnesses is considered complex. All you need to know is that you don't want to be that veterinarian that over-diagnoses hypothyroidism! Try anyway. :)


I do not diagnose a dog with hypothyroidism when they have a low total T4. Look at what their clinical signs are, if it is just that they are overweight, with no other signs. You will want to do further testing. Many laboratories will suggest a free T4 by equilibrium dialysis, so I will do that with an endogenous TSH.


It turned out that my patient was being treated for suspected hypothyroidism, where the only clinical sign was weight gain, and after the cancer was removed, the dog's thyroid gland turned to it's normal function. However, no one checked the thyroid levels after this. After taking a thorough history of what had been going on, and mentioned euthyroid sickness to the owner, and when we tested the thyroid levels, he was hyperthyroid. This is called iatrogenic hyperthyroidism. Once you start thyroid medications, due to the feedback loop to the brain to release or suppress TSH (thyroid stimulating hormone), you cannot just stop the thyroid medication as then you can have iatrogenic hypothyroidism. So wean them down slowly. The iatrogenic hyperthyroidism may also be a reason for the elevated liver enzymes. Though we cannot rule out right-sided heart failure as a cause of this as well.


It was a complicated case, with a lot of different issues that could be going on. The main point is to have some skepticism when evaluating your lab work. What are all the causes of the changes on that lab work?

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