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Q18 - The Thyroid Axis in Five Clinical Scenarios

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Endocrine System Physiology

Thyroid hormones (T3 and T4) regulate metabolism. Their release is controlled as shown in the figure. T4 can be metabolised to T3 in tissues.

Flowchart: Hypothalamus (top box) releases TRH (+) down to Anterior Pituitary (middle box), which releases TSH (+) down to Thyroid (lower box), which releases T4 and T3 (+) up from a “Peripheral body/blood” box. Minus signs mark negative-feedback arrows running from the peripheral hormone box back up to both the Anterior Pituitary and the Hypothalamus. Fig.1. Legend 1 = Hypothalamus. Legend 2 = Anterior Pituitary. Legend 3 = Thyroid. Legend 4 = Peripheral body / blood. TRH = Thyrotropin-releasing hormone. TSH = Thyroid-stimulating hormone.

Which hormone’s levels would you expect to be increased in patients with the following conditions, relative to healthy individuals?

Q18.1 (T3). Hashimoto thyroiditis (gradual immune destruction of thyroid tissue): T3 levels are increased relative to healthy individuals.
Q18.1 (T4). Hashimoto thyroiditis: T4 levels are increased.
Q18.1 (TSH). Hashimoto thyroiditis: TSH levels are increased.
Q18.1 (TRH). Hashimoto thyroiditis: TRH levels are increased.
Q18.2 (T3). Graves' disease (TSHR-stimulating antibodies): T3 levels are increased.
Q18.2 (T4). Graves' disease: T4 levels are increased.
Q18.2 (TSH). Graves' disease: TSH levels are increased.
Q18.2 (TRH). Graves' disease: TRH levels are increased.
Q18.3 (T3). A patient abusing T4 supplement pills for weight loss: T3 levels are increased.
Q18.3 (T4). T4-abuse patient: T4 levels are increased.
Q18.3 (TSH). T4-abuse patient: TSH levels are increased.
Q18.3 (TRH). T4-abuse patient: TRH levels are increased.
Q18.4 (T3). A rare TSH-secreting anterior pituitary tumour: T3 levels are increased.
Q18.4 (T4). TSH-secreting pituitary tumour: T4 levels are increased.
Q18.4 (TSH). TSH-secreting pituitary tumour: TSH levels are increased.
Q18.4 (TRH). TSH-secreting pituitary tumour: TRH levels are increased.
Q18.5 (T3). Thyroid hormone resistance (a mutated thyroid hormone receptor): T3 levels are increased.
Q18.5 (T4). Thyroid hormone resistance: T4 levels are increased.
Q18.5 (TSH). Thyroid hormone resistance: TSH levels are increased.
Q18.5 (TRH). Thyroid hormone resistance: TRH levels are increased.
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Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key