Thyroid Hormones

Serum (clotted blood)

Thyroxine (T4) is the major secretory product of the thyroid
glands (alongside a little tri-iodothyronine (T3)) and is present
in plasma as a free (active) fraction (fT4) and a protein bound
inactive fraction. fT4 is converted to fT3 which is the main
active hormone. Testing for fT3 is not currently available.
Resting concentrations of T3 or T4 rarely give useful
information as decreases are common as a consequence of
many unrelated disease processes (non-thyroidal illness
syndrome), although rare cases of hyperthyroidism might be
first identified by high resting concentrations. There are
serious doubts about the existence of hypothyroidism as a
clinical problem in horses, although a stimulation test can be
performed with 1 mg TRH iv, measuring fT3 and fT4 at 2 and 4
hours. Unfortunately some normal horses do not increase
much after stimulation so the test can be hard to interpret.

See also
TRH stimulation test (thyroid disease)