Wed17November0747PM 7

The thyroid hormones are highly plasma protein bound.

Which of the following plasma proteins does triiodothyronine (T3) bind most avidly to?

(Please select 1 option)

Thyroxine binding globulin This is the correct answer

Transthyretin

Albumin Incorrect answer selected

Serum lipoproteins

Immunoglobulins

Explanation

Thyroxine binding globulin (TBG) binds 75% of serum thyroxine (T4) and 75% triiodothyronine (T3). Quantitative and qualitative abnormalities of this protein have most profound effects on the total iodothyronine levels in serum.T3 and T4 binding to TBG is characterized by low capacity but high avidity; the converse is true, ie, high capacity but low avidity, for these hormones binding to transthyretin and albumin.

A minor proportion of the thyroid hormones are bound to serum lipoproteins. Very rarely, and in the context of anti-TH antibodies in autoimmune thyroid disease, immunoglobulins also may bind thyroid hormones. Transthyretin (thyroxine-binding prealbumin) binds 10-15% of the thyroid hormones.

Unbound T4 = 0.03%.

Unbound T3 = 0.3%.

Answer Statistics

1

69%

2

6%

3

24%

4

2%

5

1%

Times answered: 253