OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

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When is thyroxine released from the thyroid gland?

In response to adrenaline levels
When stimulated by thyroid stimulating hormone (TSH)
During periods of high metabolic activity
When blood glucose levels are low

The correct answer is: When stimulated by thyroid stimulating hormone (TSH)

Thyroxine is released from the thyroid gland primarily when stimulated by thyroid stimulating hormone (TSH). TSH, which is produced by the pituitary gland, plays a crucial role in regulating the production and release of thyroxine. When TSH is released into the bloodstream, it binds to receptors on the thyroid gland, triggering the synthesis and secretion of thyroxine. This hormone is essential for regulating metabolism, growth, and development in the body. The other options do not correctly describe the direct mechanism for thyroxine release. While factors like metabolic activity and blood glucose levels can influence overall hormone secretion rates, the specific trigger for thyroxine release is the presence of TSH in the bloodstream. Adrenaline, on the other hand, is related to the body's fight-or-flight response and does not directly stimulate thyroxine release.