Disorders of Thyroid Hormones in Rabbits
Euthyroid Sick Syndrome (ESS) and Thyroid Diseases
The thyroid gland produces hormones and has a complex influence on the rabbit’s body, affecting metabolism, circulation, and growth, among other functions. If too many or too few hormones are produced, it can have consequences for the rabbit’s health.
The main thyroid hormones include:
- Thyroxine (T4) – the transport form
- Triiodothyronine (T3) – the biologically active form
Symptoms
Too Few Thyroid Hormones (Hypothyroidism or Reduction Due to Other Diseases)
- Overweight that does not respond to diet changes
- Lethargy, reduced activity, increased need for sleep
- Hair loss, bald patches
- Constipation, intestinal obstruction (ileus), megacolon
- Slowed heartbeat, altered blood pressure
Too Many Thyroid Hormones (Hyperthyroidism – Extremely Rare, e.g., Due to Thyroid Cancer)
- Underweight despite a good appetite
- Restlessness
- Rapid heartbeat
Diagnosis and Treatment
Thyroid diseases in rabbits are still poorly researched and rarely documented in veterinary literature. Additionally, diagnosing these conditions is challenging because only the T4 value can be measured in rabbit blood. Other thyroid parameters commonly used in dogs and cats are currently not available for rabbits.
The T4 value can provide an initial indication in cases of extreme emaciation or obesity, but it is not a definitive diagnostic tool. There are no established reference values for rabbits, though one study reported a T4 reference range of 0.65–2.85 µg/dl (8.4–36.7 mmol/L) in healthy rabbits. However, it is known that thyroid hormone reference values vary significantly between laboratories, so laboratory-specific reference values should be used whenever possible.
Relying solely on T4 measurements is insufficient. Due to the limited research available and the lack of comprehensive thyroid hormone testing for rabbits, a suspected case of hypothyroidism or euthyroid sick syndrome in obese rabbits should first be addressed through dietary measures and a thorough investigation of underlying causes before considering medication.
If medication is necessary, Euthyrox (initially 10–20 µg/kg) can be administered. However, continuous monitoring is essential during treatment. Regular veterinary check-ups and additional tests are recommended to ensure proper management. Medication dosages should be adjusted based on ongoing T4 level assessments.
Distinguishing Thyroid Diseases from Euthyroid Sick Syndrome (ESS)
Euthyroid Sick Syndrome (ESS, also known as Nonthyroidal Illness Syndrome) must be differentiated from actual thyroid diseases. In ESS, the T4 hormone level drops significantly due to other non-thyroidal illnesses. A study has linked this condition to fractures, obstructive ileus, dental disease, and urinary tract disorders, though other diseases may also be contributing factors. Additionally, certain medications can temporarily cause a decline in T4 levels.
ESS is very common in rabbits. For example, a rabbit with a serious underlying condition (such as undiagnosed tumors) may show extremely low T4 levels, even though its thyroid gland is functioning normally.
There is debate among experts about how many rabbits suspected of having thyroid disease actually suffer from ESS. Interestingly, treatment with Euthyrox appears to be effective in these cases, often leading to weight reduction. However, the necessity of medication remains controversial. The primary focus should be on identifying and treating the underlying disease before considering thyroid hormone therapy.
Impact of Diet on Thyroid Hormones
Diet can also influence thyroid hormone levels. A diet high in cruciferous vegetables (such as cabbage, broccoli, kale, radish greens, and turnip greens) may inhibit iodine absorption, leading to lower T4 levels. To maintain healthy thyroid function, a balanced diet should be ensured.
Case Study: Maya
Symptoms:
Maya, a rabbit weighing 3.1 kg, exhibited:
- Obesity that persisted despite multiple diet attempts
- Lethargy
- Sore hocks (due to excess weight and inactivity)
- Hair loss
- Aggressiveness
- T4 level of 0.2 µg/dl
Treatment:
Maya was treated with Euthyrox, but the initial dosage was too high, causing severe trembling. The dosage was immediately reduced by half.
With the correct dosage, Maya experienced:
- Weight loss of 500g
- Healing of her sore hocks
- Increased energy and activity
- T4 levels improving to 0.8 µg/dl, later stabilizing at 0.5 µg/dl
However, during a routine blood test performed during surgery, a T4 level of 1.9 µg/dl indicated temporary hyperthyroidism, possibly influenced by other factors.
Although treatment came late, and some damage had already been done, Maya was able to enjoy a renewed, more active, and pain-free life in her later years.
Update:
Pathology results later revealed that Maya had a lymphoma, which was likely the underlying cause of her low T4 levels.
Sources include:
Brandao, J., Higbie, C., & Rick, M. (2015): Naturally occurring idiopathic hyperthyroidism in two pet rabbits. In Exoticscon Proceedings San Antonio (p. 341).
Brandão, J., Ellison, M., Beaufrère, H., Johnson, J., Rick, M., & Tully, T. (2014). Quantitative 99M-Technetium pertechnetate thyroid scintigraphy in euthyroid New Zealand white rabbits (Oryctolagus cuniculus). Veterinary Radiology and Ultrasound, 55(6), 689-690.
DiGeronimo, P. M., & Brandão, J. (2020): Updates on Thyroid Disease in Rabbits and Guinea Pigs. Veterinary Clinics: Exotic Animal Practice, 23(2), 373-381.
Dinges, H. P., & Kovac, W. (1972): Metastasizing thyroid carcinoma in the rabbit. Zeitschrift fur Versuchstierkunde, 14(4), 197-204.
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