**Core Concept:** Thyroid hormones, particularly thyroxine (T4) and triiodothyronine (T3), play a vital role in fetal development. Maternal thyroid hormones cross the placenta and contribute to fetal thyroid hormone levels.

**Why the Correct Answer is Right:** Thyroid hormones are essential for normal fetal development, and fetal production is insufficient to meet the needs of the rapidly growing fetus. At term, approximately 85-90% of fetal thyroxine (T4) is derived from the mother. This is due to the insufficient production of T4 by the fetal thyroid gland and the high demand for thyroid hormones during this crucial stage of development.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because fetal thyroid hormones, particularly T4, are not the primary source of fetal thyroid hormone levels at term.
B. Although maternal thyroid hormones do enter the fetal circulation, the percentage is less than the correct answer.
C. Similar to option A, fetal thyroid hormones are not the primary source of fetal thyroid hormone levels at term.
D. As mentioned earlier, maternal sources account for approximately 85-90% of fetal thyroxine, making this option partially correct but not the whole answer.

**Clinical Pearl / High-Yield Fact:** The placenta plays a crucial role in the transport of thyroid hormones from the mother to the fetus, ensuring that the fetal thyroid gland develops properly and meets its needs during the critical period of fetal development.

**Correct Answer:** D. Maternal sources account for approximately 85-90% of fetal thyroxine at term.