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CASE FILES: BIOCHEMISTRY
the oxidation of iodide ion (I-) to atomic iodine (I0), which is required for
the iodination of tyrosyl residues on thyroglobulin; these drugs decrease the
amounts of T3 and T4 synthesized. They also decrease the amount of TSIg that
is synthesized by mechanisms that are not understood. Radioactive iodine
treatments cause the progressive destruction of thyroid cells, thus reducing
thyroid function over a period of several weeks to months.
C O M P R E H E N SIO N Q U E ST IO N S
[7.1]
The thyroid hormones T3 and T4 are synthesized in the follicular cells
of the thyroid gland. From which of the following essential amino
acids are the thyroid hormones synthesized?
A. Isoleucine
B. Lysine
C. Methionine
D. Phenylalanine
E. Valine
[7.2]
The thyroid hormones T3 and T4 bind to the thyroid hormone receptor
(THR) in the target cells. Which of the following mechanisms best
describes the role of the THR?
A. It activates adenylyl cyclase to produce cAMP.
B. It activates the phosphoinositide cascade.
C. It is a soluble guanylyl cyclase.
D. It is a tyrosine kinase.
E. It is a transcription factor.
[7.3]
A 26-year-old male presents complaining of heat intolerance, heavy
sweating, tremulousness, and feeling “jittery inside.” Physical exami-
nation reveals reddened conjunctiva and warm and moist palms, but the
thyroid gland was not visibly enlarged. Which of the following tests
would be most helpful to obtain an accurate diagnosis?
A. Electrocardiogram
B. Free thyroxine level
C. Serum cortisol levels
D. Serum electrolytes
E.
Serum glucose level
A nsw ers
[7.1]
D. Thyroid hormones are synthesized from tyrosyl residues on thy-
roglobulin in the colloidal space of the thyroid follicular cells.
Although tyrosine can be obtained from the diet, it can also be syn-
thesized from the essential amino acid phenylalanine by the action of
phenylalanine hydroxylase.
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