Thyroid Test

[themule]

Question might be a NBME #6 spoiler.

In a 2 yo what's the best test for definitive thyroid function?

Why is it not Free T4?

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[sgod34]

Question might be a NBME #6 spoiler.

In a 2 yo what's the best test for definitive thyroid function?

Why is it not Free T4?

The answer should be TSH. TSH is the best initial screening test to diagnose thyroid dysfunction because it will either be elevated or decreased if there is a thyroid dysfunction. In primary HYPOthyroidism it will be elevated and in secondary it will be decreased. In primary HYPERthyroidism it will be decreased and in secondary it will be increased. If TSH is normal, T4 will be normal and thyroid function is intact, and there is no reason to even tests T4.

 

[themule]

The answer should be TSH. TSH is the best initial screening test to diagnose thyroid dysfunction because it will either be elevated or decreased if there is a thyroid dysfunction. In primary HYPOthyroidism it will be elevated and in secondary it will be decreased. In primary HYPERthyroidism it will be decreased and in secondary it will be increased. If TSH is normal, T4 will be normal and thyroid function is intact, and there is no reason to even tests T4.

Right I get all that, but what test is "most definitive of thyroid function"? Is that still TSH? Can you do a total T3 or a free T3?

 

[justdoit31]

Right I get all that, but what test is "most definitive of thyroid function"? Is that still TSH? Can you do a total T3 or a free T3?

TSH is the most definitive because if there is a thyroid problem it will be either elevated or decreased. If there is no problem it will be normal. So when you go to an endocrinologist they usually only order a TSH as the initial screen. It is definitive in that if it is normal you know that the thyroid is normal. Looking at T3 or T4 isn't a good test because some medications can effect those numbers even if the thyroid is completely normal- I believe Goljan mentioned estrogen/Oral contraceptives as an example.

As a side- I'm hypothyroid, my condition was diagnosed with a TSH and monitored with TSH alone. My specialist did run free/total T3 and T4 levels once a diagnosis was made and also checked for antibodies- but all that was after we knew 100% that my thyroid was out using TSH.