Dynamics of serum thyrotropin and thyroid hormone changes in fasting

Area of Science:

• Endocrinology
• Metabolism

Background:

• Fasting significantly impacts thyroid hormone regulation.
• The role of thyrotropin-releasing hormone (TRH) in fasting-induced thyroid changes is unclear.

Purpose of the Study:

• To investigate if decreased hypothalamic TRH secretion causes the transient decline in thyroid-stimulating hormone (TSH) during early fasting.
• To elucidate the sequence of thyroid hormone index changes during fasting.

Main Methods:

• Administered constant infusions of TRH or saline to mildly obese females during a 6-day fast.
• Monitored serum TSH, total T4 (TT4), free T4 (FT4), total T3 (TT3)/TT4, total rT3 (TrT3)/TT4 ratios, and plasma TRH levels.

Main Results:

• Fasting-induced TSH decline was not mediated by TRH.
• Plasma TRH levels remained unchanged during fasting.
• Observed a specific sequence: FT4 rise, followed by TT3/TT4 fall, then TSH decline, and finally TrT3/TT4 rise.

Conclusions:

• Early fasting's TSH suppression is due to elevated FT4, not reduced TRH.
• Fasting alters thyroid hormone indices in a specific temporal order.
• Declining serum TT3 levels correlate with TSH normalization during prolonged fasting.