A 2-year-old child with decreased serum T4 and coarse features; the most informative additional lab test would be the serum:

A 2-year-old child with decreased serum T4 and coarse features; the most informative additional lab test would be the serum:

• A. Thyroxine binding globulin
• B. Thyroid-stimulating hormone
• C. Triiodothyronine
• D. Cholesterol

Answer: (B)

The important idea here is how the pituitary-thyroid axis responds when thyroid hormone is low. When serum T4 is reduced, measuring serum TSH shows whether the thyroid gland itself is the problem or if the pituitary/hypothalamus isn’t signaling properly. If the illness is primarily within the thyroid, the pituitary responds to the low T4 by raising TSH levels. In that case, TSH will be elevated. If the issue is central—originating in the pituitary or hypothalamus—TSH may be normal or inappropriately low despite the low T4. So the serum TSH level helps distinguish primary hypothyroidism from central hypothyroidism and shapes management. The other tests don’t provide that key distinction as directly. Thyroxine-binding globulin affects total T4 measurements but doesn’t diagnose the cause of hypothyroidism. Triiodothyronine can be normal in many hypothyroid states, making it less informative. Cholesterol can be elevated in hypothyroidism but is a nonspecific finding and does not confirm the diagnosis or its cause.

The important idea here is how the pituitary-thyroid axis responds when thyroid hormone is low. When serum T4 is reduced, measuring serum TSH shows whether the thyroid gland itself is the problem or if the pituitary/hypothalamus isn’t signaling properly. If the illness is primarily within the thyroid, the pituitary responds to the low T4 by raising TSH levels. In that case, TSH will be elevated. If the issue is central—originating in the pituitary or hypothalamus—TSH may be normal or inappropriately low despite the low T4. So the serum TSH level helps distinguish primary hypothyroidism from central hypothyroidism and shapes management.

The other tests don’t provide that key distinction as directly. Thyroxine-binding globulin affects total T4 measurements but doesn’t diagnose the cause of hypothyroidism. Triiodothyronine can be normal in many hypothyroid states, making it less informative. Cholesterol can be elevated in hypothyroidism but is a nonspecific finding and does not confirm the diagnosis or its cause.