E. Thyroid hormones are similar to steroid hormones in that they bind
to receptors in the nucleus of the cell. On binding to the receptor, the
receptor-hormone complex binds to DNA affecting the transcription
of messenger RNA.
B. This patient appears to have a hyperthyroid condition even though
the thyroid does not appear to be enlarged. Thyroid function tests
would be most helpful to determine if this is the case. The free thy-
roxine level is a direct measure of the amount of free T4, the biolog-
ically active T4, in the serum. Elevation of the free T4 indicates a
B IO C H E M IS T R Y PE A R L S
Hormones bind to receptors, which can either be on the cell surface
or, as in the case of steroid and similar hormones, within the cell.
The hormone-receptor complex starts a series of events in which the
signal is converted to other chemical forms that bring about
changes in the biochemical reactions within the cell.
Hormones that bind to cell-surface receptors activate or form a com-
plex with a transducer protein in the membrane that will cause the
activation of some enzymatic activity in the cytoplasm of the cell,
producing a second messenger.
Nuclear receptors are intracellular proteins present in either the
cytosol or the nucleus that bind hormones (steroids) which cross
the cell membrane by simple diffusion. The nuclear receptors
undergo conformational changes that enable them to bind to
DNA at specific sites.
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