Discrepancy between free T4 and TSH

[ilovemacaroons]

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Hi all. Need some clarification between free T4 and TSH and which one to use in terms of diagnosing thyroid problems.

On NBME 7, 2 year old boy is brought to the physician by his mother. He has a family history of hypothyroidism and thyroiditis. He is at the lower percentile for both height and weight. His mother is concern that he is smaller than his siblings were at the same age. Mother insist to be tested for thyroid problems. Measurement of which of the following serum concentrations would provide the most definitive evidence for normal thyroid function for the son?

A. FT4
B. Iodine
C. TSH
D. Total T4
E. Total T3

On NBME 17, a 28 year old woman at 18 weeks pregnant has palpitations. Lab shows increased serum total T4 concentration. Which of the following tests is best to confirm hyperthyroidism?
A. Fine-needle biopsy of the thyroid.
B. Free T4
C. Radioactive iodine uptake.
D. Serum T3
E. Thyroid Ab

My question is, if there was an answer choice of TSH for NBME 17, would that be a better answer than free T4? Also, why wouldn't total T3 also be a good choice for NBME 17? Thanks!

 

[TBV]

My only hesitation about TSH in the pregnant woman is the effects of HCG on the thyroid. Might not give you the most accurate representation of what is really going on if you get TSH.

Also T3 is primarily generated at the tissue level so in general not a great test as it's difficult to assess what is going on peripherally.

 

[pacman2018]

The way I think of it is to start with TSH. T4 levels can be affected by a lot of different factors. For the second question, I assume the answer is free T4; in pregnancy, thyroid binding globulin is increased. Therefore, to have a normal free T4 level, total T4 would have to increase.