**Core Concept**
Childhood hypothyroidism, often caused by congenital hypothyroidism or autoimmune thyroiditis, requires lifelong treatment to replace thyroid hormone and prevent growth and developmental delays. The primary goal of treatment is to maintain normal levels of thyroxine (T4) and triiodothyronine (T3) in the body.

**Why the Correct Answer is Right**
Levothyroxine (T4) is the first-line treatment for childhood hypothyroidism. This synthetic thyroid hormone is administered orally, usually once daily, and is well-absorbed by the gut. The dosage is adjusted based on regular measurements of serum TSH and free T4 levels to ensure optimal thyroid hormone replacement. This targeted approach helps prevent complications such as growth retardation, delayed puberty, and cognitive impairment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Liothyronine (T3) is not the first-line treatment for childhood hypothyroidism. While T3 is an active thyroid hormone, it has a shorter half-life and is more likely to cause side effects when used alone.
**Option B:** This option is incorrect because Methimazole is an antithyroid medication used to treat hyperthyroidism, not hypothyroidism. It works by inhibiting the synthesis of thyroid hormones.
**Option C:** This option is incorrect because Thyroglobulin is a protein produced by the thyroid gland, but it is not used as a treatment for hypothyroidism.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor serum TSH and free T4 levels regularly in children with hypothyroidism to adjust the levothyroxine dosage and prevent over- or under-treatment.

**Correct Answer:** L. Levothyroxine