Keynote Message

Iatrogenic hypothyroidism is common following treatment for hyperthyroidism. Until recently, this fact has been largely ignored, primarily because the patients are usually clinically well and do not demonstrate overt manifestations of hypothyroidism. Cats with iatrogenic hypothyroidism are more likely to be azotaemic than those that remain euthyroid following treatment. Furthermore, as might be anticipated, restoring euthyroidism in cats with iatrogenic hypothyroidism results in decreases in creatinine concentration in the majority, and resolution of azotaemia in a proportion. It has been observed that while in hyperthyroid cats overall, development of azotaemia following treatment for hyperthyroidism does not alter prognosis, this does make a difference in cats with iatrogenic hypothyroidism with those cats with both hypothyroidism and azotaemia having shortened survival times. This has led to interest in tailoring treatment to avoid development of iatrogenic hypothyroidism. Unfortunately, with this approach there may be a trade-off with an increased risk of unsuccessful/incomplete treatment of hyperthyroidism.

In this session, we will discuss whether approaches to avoid iatrogenic hypothyroidism are justified and also whether or not they are likely to be successful. This will be considered for all treatment modalities (medical, diet, surgery and I-131). Criteria for confirming the diagnosis of iatrogenic hypothyroidism will be discussed, and recent literature reviewed to consider whether iatrogenic hypothyroidism is likely to be transient or permanent, and the implications of this on the decision whether or not treatment is indicated.

Key References

1.  Williams TL, Elliott J, Syme HM. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. Journal of Veterinary Internal Medicine. 2010;24:1086–1092. DOI: 10.1111/j.1939-1676.2010.0566.x.

2.  Williams TL, Elliott J, Syme HM. Effect on renal function of restoration of euthyroidism in hyperthyroid cats with iatrogenic hypothyroidism. Journal of Veterinary Internal Medicine. 2014;28(4):1251–5. DOI: 10.1111/jvim.12359.

3.  Kahraman D, Keller C, Schneider C, et al. Development of hypothyroidism during long-term follow-up of patients with toxic nodular goitre after radioiodine therapy. Clinical Endocrinology 2012;76:297–303. DOI: 10.1111/j.1365-2265.2011.04204.x.

4.  Peterson ME. Feline focus: diagnostic testing for feline thyroid disease: hypothyroidism. Compendium on Continuing Education for the Practicing Veterinarian. 2013;35(8): E1–6. DOI: PMID: 23975586.

5.  Peterson ME, Broome MR. Ultra-low doses of radioiodine are highly effective in restoring euthyroidism without inducing hypothyroidism in most cats with milder forms of hyperthyroidism: 131 cases. Journal of Veterinary Internal Medicine. 2014;28(3):1031. DOI: 10.1111/jvim.12361.