Hypophysectomy abolishes rhythms in rat thyroid hormones but not in the thyroid clock

Area of Science:

• Endocrinology
• Chronobiology
• Molecular Biology

Background:

• Circadian rhythms regulate endocrine functions, including the hypothalamic-pituitary-thyroid axis.
• Daily rhythmicity of thyroid-stimulating hormone (TSH) is known, but the existence and function of a local thyroid clock remain unclear.

Purpose of the Study:

• To investigate the presence and function of a thyroid clock in rats.
• To determine the regulation of daily thyroid hormone rhythms.

Main Methods:

• Assessed daily oscillations of clock gene mRNA (Per1, Bmal1) and PER1 protein shuttling in rat thyroid tissue.
• Examined circulating thyroid hormones and TSH levels in intact and hypophysectomized rats.
• Analyzed TSH receptor mRNA expression.

Main Results:

• Thyroid clock genes Per1 and Bmal1 exhibited 24-h antiphase oscillations, unaffected by hypophysectomy.
• PER1 protein showed daily cytoplasmic-nuclear shuttling in thyroid cells.
• Normal rats displayed daily rhythms in thyroid hormones, preceded by TSH peaks.
• Hypophysectomy abolished thyroid hormone oscillations and lowered levels, despite the intact thyroid clock.
• No daily oscillations in TSH receptor mRNA were observed.

Conclusions:

• A functional thyroid clock exists in rats, independent of the pituitary.
• Daily thyroid hormone secretion rhythm is primarily controlled by the suprachiasmatic nucleus (SCN) via rhythmic TSH secretion, not by the local thyroid clock.