Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1247 page)

   Hypothalamic hypothyroidism: low serum T
₃
, T
₄
, and TSH levels, with a TRH response that may be exaggerated or normal or (most characteristically) with a peak delay 45–60 minutes

   TSH high sensitivity, <0.1 mU/L, obviates need for TRH, except for TSH-secreting tumor and thyroid hormone resistance (in which case TSH thyroxine is high).

   Interpretation must be based on clinical studies that exclude the pituitary gland as the site of the disease.

   Lack of response shows adequate therapy in patients receiving thyroid hormones to shrink thyroid nodules and goiters and during long-term treatment of thyroid carcinoma.

   In patients with euthyroid Graves disease who have only exophthalmos (unilateral or bilateral), the TRH stimulation test may sometimes be normal. A T
₃
suppression test may be required.

   Elderly patients with or without symptoms of hyperthyroidism may have serum T
₄
and T
₃
in upper normal range.

   Euthyroid sick syndrome—response varies. Some patients respond normally, whereas many have less than normal response.

   Limitations

   Contraindicated in pregnancy.

   No T
₄
or T
₃
should be given for 3 weeks prior to test.

   TRH can cause smooth muscle spasm; use with caution in asthma and ischemic heart disease.

   The TSH response to TRH is modified by antithyroid drugs, corticosteroids, estrogens, large amounts of salicylates, and levodopa.

THYROXINE, FREE (FT
₄
)

   Definition