Q19 - Thyroid Function Tests: TSH, T4, and T3RU

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Thyroid hormones are transported in blood by proteins. The TBG (thyroid hormone binding globulin) is the main thyroid hormone transporting protein. Many factors affect TBG concentration such as oestrogen and OCP (oral contraceptive pill), etc. OCP increases TBG concentration. T3RU (T3 resin uptake) assay is a method of quantification of unbound TBG in the blood. It indirectly measures the capacity of patients TBG to bind radioactive labelled T3. The less patients thyroid hormone level is the more radioactive labelled T3 will bound to TBG. Thus, since radioactive T3 is only detectable in unbound form, the T3RU assay result will be lower.

Using the information and data, determine which of the statements are true or which are false.

A. In primary hypothyroidism, the thyroid function test would be like below: TSH increased, T4 decreased, T3RU decreased.

B. A person who uses OCP and is euthyroid (normal functioning of thyroid) the thyroid function test would be like below: TSH normal, T4 decreased, T3RU decreased.

C. In primary hyperthyroidism the thyroid function test would be like below: TSH increased, T4 Increased, T3RU increased.

D. In secondary hypothyroidism (pituitary dysfunction) the thyroid function test would be like below: TSH decreased, T4 decreased, T3RU decreased.

E. In tertiary hypothyroidism (hypothalamic dysfunction) the thyroid function test would be like below: TSH normal, T4 decreased, T3RU decreased.

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Question reproduced from IBO 2018, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)