Embryonic exposure to excess thyroid hormone causes thyrotrope cell death

Area of Science:

• Endocrinology
• Developmental Biology
• Comparative Physiology

Background:

• Central congenital hypothyroidism (CCH) is linked to maternal hyperthyroidism, suggesting thyroid hormone's role in fetal thyroid axis development.
• The zebrafish embryo serves as a model to study the ontogeny of thyroid regulation and the impact of thyroid hormone on pituitary development.

Purpose of the Study:

• To characterize the development of negative feedback regulation in the thyroid axis.
• To investigate the effects of excess thyroid hormone on thyrotrope development and function in zebrafish.

Main Methods:

• Utilized zebrafish embryos to study thyroid axis development.
• Co-expression analysis of thyroid-stimulating hormone β subunit (tshb) and type 2 deiodinase (dio2) in thyrotropes.
• Employed transgenic zebrafish (GFP under tshb promoter) for in vivo thyrotrope fate tracking.
• Conducted time-lapse imaging to observe the effects of elevated thyroid hormone.

Main Results:

• Thyroid-stimulating hormone β subunit (tshb) and type 2 deiodinase (dio2) are coexpressed in thyrotropes by 48 hours post-fertilization.
• Thyroid hormone-driven negative feedback on tshb transcription is established by 96 hours post-fertilization, correlating with increased systemic TH levels.
• Early exposure to elevated TH induces thyrotrope cell death, with slow recovery upon TH removal.

Conclusions:

• Transient thyroid hormone exposure significantly impacts the thyrotrope population during critical pituitary development.
• These findings suggest potential long-term implications for thyroid-stimulating hormone (TSH) production reserve and set point.
• The study highlights the sensitivity of the developing thyroid axis to thyroid hormone levels.