Imagine this clinical scenario: the owner of a geriatric cat diagnosed with hyperthyroidism is seeking non-invasive and simple treatment. Traditionally, treatment may involve a choice between surgery, lifelong medical management or radioactive iodine treatment. Surgery is invasive, irreversible and carries the risk of anaesthetising a geriatric patient that may have associated cardiac changes and frequently other concurrent conditions such as chronic kidney disease. Traditional medical management may result in owners having to give tablets daily, or twice daily, potentially to a non-compliant cat. Radioactive iodine treatment is only available at limited centres and means the owner will have to leave their cat at the centre for several weeks. You have read about the use of an iodine-restricted diet to reduce the severity of clinical signs of hyperthyroidism and normalise serum total thyroxine (TT4) levels. If it works, a simple dietary change would be a useful way to manage the disease – so you investigate the evidence.
Five studies were critically appraised: three prospective studies, one randomised controlled study and one retrospective case series.
The first prospective study investigated 225 hyperthyroid cats who were fed solely on an iodine-restricted diet (van der Kooij et al., 2014). Group one was previously medicated with antithyroid medications, group two was untreated. Outcomes studied include body condition score, clinical signs scored by the owners and vets at day 0, week 4 and week 8, and an objective assessment of TT4, urea and creatinine. Serum TT4 was normalised in 75 percent of cats by day 56; in the remaining subjects, serum TT4 was lower but still above the reference range. Clinical signs were perceived to have improved by both owners and veterinarians, and body condition score stabilised. Limitations include missing data due to voluntary participants and weight was monitored subjectively using body condition scores rather than scales.
One randomised controlled study investigated 22 cats with suspected hyperthyroidism, 12 of which were fed on a prototype restricted iodine diet and 10 were fed a control diet (Fritsch et al., 2014). Cats were assessed over 12 weeks. All cats fed the iodine-restricted diet had decreased TT4 concentrations by the end of the study, with half attaining euthyroid status. In the control group, TT4 was increased in four cats, reduced in four cats and stable in two. This study was funded and designed by the producer of the iodine-restricted diet, and contained relatively small sample sizes.
In a further prospective study, 15 cats with hyperthyroidism were fed an iodine-restricted diet and were assessed for up to six months (Vaske et al., 2016). TT4 was reduced in 100 percent of the cats after six months, though 9 to 10 cats remained persistently hyperthyroid after six months. Limitations include that the study was funded and designed by Hill’s Pet Nutrition, producer of the iodine-restricted diet, and the sample size was small.
In the final prospective study, eight cats were fed on an iodine-restricted diet for six months (Scott-Moncrieff et al., 2015). Clinical signs were reportedly resolved, and both TT4 and fT4 were significantly improved, with seven out of eight cats achieving euthyroid status by week eight. Limitations include the very small sample size and no standardised assessment for clinical signs was described.
In a retrospective case series, 49 cats with hyperthyroidism were fed solely on an iodine-restricted diet (Hui et al., 2015). Serum TT4 was normalised in 83 percent of cats by 180 days. Normalisation of TT4 did not lead to any improvement of weight or heart rate; however, weights did stabilise and clinical signs did not worsen. Limitations include that nine cats had already been treated using methimazole and the measurements were only taken for eight weeks. Retrospective case studies are also not considered high-quality evidence and are prone to several types of bias.
Overall, the limited evidence suggests that iodine-restricted diets can help to reduce levels of serum TT4 in hyperthyroid cats, as all the studies found that 100 percent of the cats had lower serum TT4 values at the end of the follow-up period than at the start of the study. However, this did not always guarantee a return to euthyroid status, and there is a lack of consensus regarding the resolution of clinical signs. No cats were removed from the studies due to safety concerns or side effects, so the diet appears to be safe.
A proportion of the studies were undertaken in conjunction with a commercial pet nutrition company, who is a key developer and producer of commercially available iodine-restricted diets, so potential bias should be considered.
A limitation of all the studies is the difficulty in maintaining and proving 100 percent compliance, as all the patients enrolled in the studies were client-owned and potentially allowed outdoor access so therefore could be feeding elsewhere. Iodine can also be present in tap water at variable levels. The limited follow-up period for all the studies further reduces the value of any conclusions that can be made.
Further research investigating iodine-restricted diets is required and studies involving larger sample sizes, with lifelong follow-up, would improve the evidence base greatly.
The full Knowledge Summary can be read in RCVS Knowledge’s open access journal Veterinary Evidence.