scott@vtx-cpd.com

Hey.

I have just seen this paper in JSAP and thought about your question above:

“Recombinant human thyrotropin stimulation test in 114 dogs with suspected hypothyroidism: a cross‐sectional study

Objective
To evaluate the performance and define cut‐offs for the interpretation of a thyroid‐stimulating hormone (TSH) stimulation test with a recombinant human TSH dose of 75 μg/dog administered intravenously in dogs with suspected hypothyroidism.

Materials and Methods
Cross‐sectional study. Medical records of dogs presented for suspected hypothyroidism were retrospectively reviewed. Animals were included if a TSH stimulation test with a recombinant human TSH dose of 75 μg/dog was performed and follow‐up was available. Dogs with a post‐TSH serum total thyroxine (T4) level of ≥2.2 μg/dL were considered euthyroid. Dogs with a post‐TSH T4 level of <2.2 μg/dL were classified as hypothyroid or euthyroid based on follow‐up, including response to levothyroxine supplementation. A receiver operating characteristic curve analysis was used to define the performance of the test.

Results
One hundred and fourteen dogs were included. Forty were classified as hypothyroid and 74 as euthyroid. Post‐TSH T4 cut‐offs of 1.3 and 1.7 μg/dL showed sensitivities of 92.5 and 100% and specificities of 97.3 and 93.2%, respectively. Post‐TSH T4 levels of >1.7 μg/dL had a negative predictive value of 100%. Post‐TSH T4 levels of <1.3 μg/dL showed a positive predictive value of 94.9%. Area under the ROC curve for post‐TSH T4 was 0.99.

Clinical Significance
A TSH stimulation test performed with a recombinant human TSH dose of 75 μg/dog is highly reliable to discriminate between hypothyroid and euthyroid dogs, even in cases of concurrent non‐thyroidal illness or administration of medications. A post‐stimulation T4 concentration of >1.7 μg/dL is suggestive of normal thyroid function.”

This might be a good option for a tricky case that is on phenobarbitone!

Scott 🙂

Hey.

Hope you are well.

Do let me know how you get on with this case.

Scott 🙂

Hey.

Hope you are having a good week and enjoying the Scottish sun! Was on Troon beach yesterday… glorious!

Sorry about the delay with this one. I hope you are safe and well. Side effects are obviously a possibility at any time, but we often see some of the more severe side effects in the first few weeks. I probably do full haematology/biochemistry at the 3-4 week mark. The thing you might not see with external signs would be the haematology changes. After that you could possibly limit to T4/urea/creatinine for the next check. There is definitely no need to do it all each time, but I probably would that first check.

We would only do bilateral surgery if both glands increased in size. I would warn owner that further surgery may be required at a later date. The calcium needs to be monitored carefully. If we feel the parathyroid has been preserved then I would not always start routinely but monitor calcium really carefully. With bilateral removal and uncertainty over parathyroid, totally reasonable to start vitamin D. I would still be keeping a close eye on the calcium as the hypocalcaemia is normally transient.

Hope that helps.

Scott x

No problem.

It really can vary.

These cats are in isolation initially so no blood tests are being done. We can see hypothyroidism within a few weeks, but it can be months down the line when this develops.

Hope you have a lovely week.

Scott 🙂

Areti,

I hope you are safe and well. I always look forward to your questions. Thank you for them!

2 – Regarding the initial dose, there is not a definite right or wrong answer. I can understand why people take the approach of starting with a higher dose when the T4 is over 100. However, there is very rarely a desperate rush to get the T4 normal. I still think it is better to take a more cautious approach and start with a lower dose and increase depending on response.

3 – There are definitely some patients that will stabilise on a 2.5mg AM and 5mg PM dose. So yes, if you are dose reducing it is totally reasonable to reduce one of the daily doses before the other. The EOD dosing makes less sense pharmacology-wise, but if it works then it is also not unreasonable! I would certainly not reduce any further than that.

4 – Life-threatening adverse reactions include agranulocytosis, thrombocytopenia, severe hepatopathy, and non-thrombocytopenia-associated bleeding. If any of these reactions occurs, the anti-thyroid drug used must be stopped immediately. The mortality rate with these adverse drug reactions is high. These severe adverse reactions seen with methimazole therapy are not reported with carbimazole, but they can still happen. It makes little sense that adverse reaction profiles would be very different between the two drugs because carbimazole is metabolized to methimazole in order to exert its therapeutic effect. So, carbimazole could be tried, with the owner being very aware of the possible risks. It is difficult to say how low is too low with lymphocytes. If the lymphocytes are the only cell line affected, I would not overly worry.

5 – When side-effects that are not life-threatening (GI) necessitate cessation of an anti-thyroid drug, reactions to later re-challenge with the drug are not predictable. I would treat the GI signs symptomatically (maropitant and probiotic) and consider oral therapy after a break. Again, as long as the owner is well informed.

6 – Even when cats develop hypocalcemia after surgery, there is still a good chance that they will not require lifelong treatment. They will often be hypocalcemia initially but this will resolve. They need to be closely monitored. It is difficult to predict how quickly the calcium will normalise. It is not unreasonable to start calcium and vitamin D therapy in bilateral cases. If the surgeon was confident to preserve the parathyroid glands then possibly not and close monitoring.

7 – Did you mean hypocalcemia with this question?

Hope that helps. Really appreciate all of your questions and support of vtx.

Scott 🙂

Hello lovely Inga,

I hope you are safe and well. I have popped some thoughts below:

1- I generally advise owners to inject as or after a dog is eating. If for some reason they have an off day and don’t eat or we are intentionally starving prior to surgery etc I advise to carry on but inj with half dose of insulin. was curious if you have a similar or different approach.

I would totally agree. We talk to our anaesthesia team a lot about this too. I would agree with the half dose if starving for a procedure or not eating for any other reason. I think it is important that if they are ‘sick’ for another reason and then do not eat, some insulin (1/2 dose) is still important. Obviously, prolonged sickness and anorexia would require investigation.

2- Have a couple of cases with very fussy toy breeds who were previously grazers. I have really struggled to get them to eat a 50% daily food ration in the morning to get their insulin and have one situation currently where the owner hand feeds some of the am ration to be able to inject then the dog has more at lunch and then normal 50% dinner and second injection. It means I cannot accurately check a trough BG level through the day but clinically the dog seems better with this approach. I just wondered for these fussy dogs do ever give a different insulin dose am and pm if meal rations is not equally split and should we relax a bit and just let them graze and inject 50:50 am and pm dose regardless…

This is a really interesting one. I certainly have seen a split/different dose regime used. I thnk it is hard to accurately dose insulin in this way. I think that as long as the dog is well clinically and DM signs have reduced/controlled, then I would allow them to graze! I would treat them more like a cat. This may not get the best control, but we have to be sensible too!

Scott 🙂