In secondary hypothyroidism, low circulating thyroid hormone levels are due to failure of TSH output as a result of pituitary disease (Fig. 15.31). The T4 levels are low, as are the basal TSH levels. There is no TSH response to exogenous TRH in severe pituitary failure, but a blunted TSH response may still occur in less-severe pituitary disease. The T3 levels may be normal despite low T4 levels, as in primary hypothyroidism, therefore the measurement of T3 is not a good diagnostic test for hypothyroidism of any origin. Some patients have mildly elevated TSH levels (6ΓÇôl0┬╡u/ml). This paradox is explained by the finding that the TSH has low biological activity, since the normal thyrotrophs are not being stimulated by TRH, and the TSH is abnormally glycosylated.
