# Assessment of the Thyroid Profile in the Iberian Lynx (Lynx pardinus)

## TL;DR

This study establishes baseline thyroid hormone levels in Iberian lynxes, showing differences between captive and wild populations.

## Contribution

The first reference intervals for thyroid hormones in Iberian lynxes, comparing captive and wild individuals.

## Key findings

- Captive lynxes had higher TT4 levels than wild lynxes.
- Captive male lynxes showed higher TT4 than females.
- Age did not significantly affect thyroid hormone levels.

## Abstract

The Iberian lynx (Lynx pardinus), once considered the world’s most endangered wild cat, has made an impressive recovery thanks to strong conservation efforts. Over the past two decades, researchers have collected extensive health data to guide management and protect the species. However, until now, thyroid function had never been studied. This research provides the first reference values for thyroid hormones in Iberian lynx, comparing animals living in captivity with those in the wild. Blood samples were collected from lynxes of different ages and both sexes. The study found that the environment (captive or wild) and sex, in the case of captive animals, could influence thyroid activity. Age, on the other hand, did not appear to play a role. By establishing these baseline values, clinicians now have an important tool to monitor the health of the Iberian lynx more effectively. Understanding thyroid function adds a new dimension to conservation work, helping ensure that this iconic species continues its path toward survival and long-term stability.

The Iberian lynx (Lynx pardinus), once considered the most endangered felid, has shown demographic recovery thanks to intensive conservation measures. Over the past two decades, large physiological datasets have supported both clinical management and research, yet thyroid function has remained comparatively underexplored. This study provides the first reference intervals (RI) for total thyroxine (TT4) and thyroid-stimulating hormone (TSH) in captive and wild lynxes, and assesses the effects of age, sex, and environment. Serum samples from 71 individuals (32 captive, 39 wild; 32 females, 39 males) were analysed, including 32 young adults, 14 adults, and 25 geriatric animals. TT4 was measured by enzyme immunoassay, and TSH using chemiluminescence. TT4 RI was 0.80–2.00 µg/dl (captive) and 0.70–2.20 µg/dl (wild) (p = 0.065). TSH RI was 0.00–1.10 ng/ml (captive) and 0.00–0.10 ng/ml (wild), showing a significant difference (p < 0.001). Captive males had higher TT4 than females (p = 0.018), while no sex difference appeared in wild lynxes (p = 0.408). Age had no significant effect on TT4 (p = 0.462) or TSH (p = 0.739). Findings confirmed that environment and sex (captivity) influenced thyroid parameters, while age did not. These values are crucial for health monitoring and endocrine assessment in this endangered species.