TRH Stimulation Thyroid

Can be used to investigate suspected hypothyroidism and
hyperthyroidism. However, there are serious doubts about whether
hypothyroidism even exists in horses, with no well-defined published
cases, and also the reference intervals might preclude proper
interpretation (see below). Although well-defined cases of
hyperthyroidism have been described in a few elderly horses showing
weight loss, tachycardia and hyperexcitability, diagnosis can usually be
based on baseline thyroid hormone levels and clinical signs.
Protocol:

1. Collect baseline serum sample for measurement of T3 and T4
   concentration (preferably free fractions although total is
   acceptable)
2. Inject 1 mg TRH IV
3. Collect further serum samples at 2 hours and 4 hours post-TRH.
   Interpretation:
   It is often suggested that, in normal horses, T3 concentrations should
   double at 2 hours and T4 concentrations should double at 4 hours.
   However, studies indicate that responses in normal horses may range
   from 1.1–10.3 times for total T3 and 1.0–53.0 times for free T3 at 2
   hours; and from 1.3–3.8 times for total T4 and 1.1–2.1 times for free
   T4 at 4 hours – meaning hypothyroidism is hard to define.

As subnormal responses to TRH may characterise both hypo- and
hyper-thyroidism, this creates diagnostic difficulties given that normal
horses may show minimal, if any, response too!

See also:
Thyroid hormones