Relationship between thyroid function and seasonal reproductive activity in mares.
Abstract: The relationship between thyroid function and seasonal reproductive activity in mares was investigated by comparing plasma triiodothyronine and thyroxine concentrations in anoestrous mares and in mares with cyclic ovarian activity during the anovulatory season. In study 1, the diurnal changes in plasma triiodothyronine and thyroxine concentrations on a single day were compared in anoestrous and cyclic mares. The mean thyroxine concentrations were significantly higher (P < 0.001) in the cyclic mares than in the anoestrous mares, whereas mean triiodothyronine concentrations were similar in both groups. In study 2, plasma thyroid hormone concentrations were monitored in mares that foaled before the vernal equinox. Nine mares resumed cyclic ovarian activity shortly after parturition and ten mares remained in anoestrus. Mean plasma triiodothyronine and thyroxine concentrations were significantly lower in the first 60-90 days after foaling in the mares undergoing anoestrus than in the mares undergoing normal ovarian activity after parturition (P < 0.001). In these mares, the increases in triiodothyronine and thyroxine concentrations were associated with the first ovulation after parturition. In study 3, thyroid hormone concentrations were monitored in adult (n=14) and young (n=14) mares. Mean thyroxine concentrations were significantly lower in adult anoestrous mares during the anovulatory period than in cyclic mares and in anoestrous mares before and after the anovulatory period (P < 0.01). Mean plasma triiodothyronine concentrations were also significantly lower (P < 0.001) in anoestrous mares from December until March. There were no significant differences between the plasma triiodothyronine and thyroxine concentrations in young cyclic and anoestrous mares. This relationship between thyroid function and expression of seasonal reproductive activity does not indicate that the thyroid gland is involved in the control of seasonality in horses. The decreased triiodothyronine and thyroxine concentrations in anoestrous mares may be the result of a hypothalamic control similar to that described for seasonal reproductive activity.
Publication Date: 2000-01-01 PubMed ID: 20681128
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- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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This research investigates the link between thyroid function and reproductive activity in horses. It specifically compares thyroid hormone levels among mares (female horses) exhibiting different reproductive behaviors during the non-breeding season.
Summary of Research Methods
- The study was divided into three parts. In each part, the researchers measured the plasma (blood) levels of two thyroid hormones, triiodothyronine (T3) and thyroxine (T4), in mares under different reproductive states or conditions (aneostrous versus cyclic).
- In Study 1, the researchers compared T3 and T4 levels over a single day in both aneostrous (non-oestrous or non-reproductive period) and cyclic (active reproductive period) mares.
- In Study 2, the researchers tracked T3 and T4 levels in mares that had given birth before the vernal equinox (the beginning of spring) and analyzed the hormonal levels in relation to the mare’s shift into aneostrus or a return to cyclic reproductive activity.
- In Study 3, the researchers compared T3 and T4 levels in adult aneostrous mares during the non-breeding period to those levels found in cyclic mares and in pre- and post-non-breeding period aneostrous mares. Additionally, they compared T3 and T4 levels in young cyclic and aneostrous mares.
Key Findings
- Results showed that cyclic mares had higher mean levels of T4 compared to aneostrous mares, while T3 levels were similar in both groups.
- In mares that had given birth before the start of spring, those which remained in aneostrus after birth had significantly lower T3 and T4 levels in the initial 60-90 postpartum days compared to mares that resumed cyclic activity. The first ovulation after birth was associated with an increase in T3 and T4 levels.
- Adult aneostrous mares during the non-breeding period had significantly lower T4 levels than cyclic mares and aneostrous mares outside the non-breeding period. Additionally, T3 levels were significantly lower in adult aneostrous mares during December to March period. Young cyclic and aneostrous mares, however, showed no significant differences in their T3 and T4 levels.
Conclusions
- Despite the observed correlations, the researchers concluded that these findings do not point to the involvement of the thyroid gland in controlling seasonal reproductive activity in horses.
- The decreases in T3 and T4 levels in aneostrous mares could be triggered by hypothalamus-related controls, similar to those explained for seasonal reproductive activity.
Significance
- This research contributes valuable insights into the complex dynamics between thyroid function and reproductive behavior in mares, enhancing understanding and informing management practices in horse breeding and care.
Cite This Article
APA
Huszenicza G, Nagy P, Juhász J, Kóródi P, Kulcsár M, Reiczigel J, Guillaume D, Rudas P, Solti L.
(2000).
Relationship between thyroid function and seasonal reproductive activity in mares.
J Reprod Fertil Suppl(56), 163-172.
Publication
Researcher Affiliations
- University of Veterinary Science, István u. 2., 1078, Budapest, Hungary.
MeSH Terms
- Animals
- Estrous Cycle / blood
- Estrous Cycle / physiology
- Female
- Horses / blood
- Horses / physiology
- Progesterone / blood
- Seasons
- Thyroid Gland / metabolism
- Thyroxine / blood
- Thyroxine / metabolism
- Time Factors
- Triiodothyronine / blood
- Triiodothyronine / metabolism
Citations
This article has been cited 3 times.- Tangyuenyong S, Nambo Y, Nagaoka K, Tanaka T, Watanabe G. Sensitive radioimmunoassay of total thyroxine (T4) in horses using a simple extraction method. J Vet Med Sci 2017 Jul 28;79(7):1294-1300.
- Mayahi S, Mamouei M, Tabatabaei S, Mirzadeh K. Reproductive characteristics and thyroidal function in relation with season in Khuzestan buffalo (Bubalus bubalis) bulls. Vet Res Forum 2014 Summer;5(3):201-5.
- Satué K, La Fauci D, Medica P, Damiá Gímenez E, Cravana C, Fazio E. Shifts between pro-inflammatory and anti-inflammatory profiles in pregnant mares: a review of physiological functions. Front Vet Sci 2025;12:1660759.
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