Effect of blocking T4-monodeiodination on hatching in chickens

Area of Science:

• Endocrinology
• Developmental Biology
• Comparative Physiology

Background:

• Thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are crucial for vertebrate development.
• The conversion of T4 to T3, known as monodeiodination, is a key regulatory step in thyroid hormone action.
• Understanding these processes in avian embryos provides insights into vertebrate development.

Purpose of the Study:

• To investigate the role of peripheral T4 to T3 conversion in late-stage chick embryos.
• To determine the effects of blocking this conversion on embryonic development and hatching.
• To examine the physiological responses to inhibited T4 to T3 conversion.

Main Methods:

• Chick embryos were injected with iopanoic acid (IOP), a known inhibitor of T4 to T3 conversion, on days 17 or 18 of incubation.
• Serum and thyroid gland levels of T3, T4, and reverse T3 (rT3) were measured using radioimmunoassays.
• Measurements were taken from day 19 through hatching and one day post-hatching.

Main Results:

• Iopanoic acid effectively blocked T4 to T3 conversion in chick embryos from day 19 onwards.
• Inhibition of T4 to T3 conversion did not significantly delay hatching or increase embryonic mortality.
• Serum levels of T4 and reverse T3 (rT3) increased after IOP administration, with the T4 rise not solely attributable to reduced conversion.

Conclusions:

• Peripheral monodeiodination of thyroid hormones occurs in the late chick embryo.
• The conversion of T4 to T3 is not essential for successful hatching or normal embryonic development in the late stages.
• Chick embryos exhibit compensatory mechanisms, such as increased T4 levels, when T4 to T3 conversion is inhibited.