In a foetus, thyroxine secretion begins by?
**Core Concept**
Thyroxine secretion in the fetus is a crucial aspect of thyroid hormone development, which plays a vital role in growth and maturation. The initiation of thyroxine secretion in the fetus is closely linked to the development of the thyroid gland and the hypothalamic-pituitary-thyroid axis.
**Why the Correct Answer is Right**
Thyroxine secretion in the fetus begins around 10-12 weeks of gestation, primarily due to the stimulation of the hypothalamic-pituitary-thyroid axis. This axis is triggered by the increase in fetal thyroid-stimulating hormone (TSH), which is produced by the pituitary gland in response to the release of thyrotropin-releasing hormone (TRH) from the hypothalamus. The production of TRH is essential for the initiation of thyroxine secretion in the fetus.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately represent the timing of thyroxine secretion in the fetus. While the thyroid gland begins to develop early in fetal life, the actual secretion of thyroxine does not commence until around 10-12 weeks of gestation.
**Option B:** This option is incorrect because it suggests that thyroxine secretion in the fetus is solely dependent on maternal thyroid hormones. While maternal thyroid hormones do cross the placenta and contribute to fetal thyroid hormone levels, the initiation of thyroxine secretion in the fetus is an autonomous process that is not solely dependent on maternal hormones.
**Option C:** This option is incorrect because it implies that thyroxine secretion in the fetus is initiated by the placenta. While the placenta does play a role in fetal thyroid hormone development, the initiation of thyroxine secretion is primarily driven by the development of the hypothalamic-pituitary-thyroid axis.
**Clinical Pearl / High-Yield Fact**
It's essential to note that congenital hypothyroidism can occur if there is a deficiency in thyroid hormone production or if the hypothalamic-pituitary-thyroid axis is not properly developed. Early detection and treatment of congenital hypothyroidism are critical to prevent long-term developmental and cognitive deficits.
**Correct Answer:** C.