Hypothyroidism Secondary to Biologically Inactive Thyroid-Stimulating Hormone Secretion by a Pituitary Chromophobe Adenoma

Abstract

A 50-year-old man, suffering from a large pituitary adenoma and panhypopituitarism, was found to have severely elevated thyrotrophin (thyroid-stimulating hormone [TSH]) levels (greater than 20.2 microunits/mL). The thyroxine (T4) level was low (less than 3.0 micrograms/dL). Thyroid sodium iodide I 131 uptake was low (5% at 24 hours). A TSH test result was normal, with a 24-hour 131I uptake of 52% and a normal-looking thyroid gland on scintiscan. After surgical removal of the pituitary chromphobe adenoma, T4 levels returned to normal (6.8 micrograms/dL) and TSH levels improved substantially (9.0 microunits/mL). Findings from repeated 131I uptake tests were normal (22% at 24 hours). Other pituitary functions improved also. These results suggest that the patient had biologically inactive TSH produced by the tumor. Removal of the tumor probably enabled recovery of the active TSH with the return of normal thyroid uptake and T4 production. Whenever hypothyroidism and high levels of TSH coexist with pituitary dysfunction, a TSH test is needed to distinguish between primary hypothyroidism and hypothyroidism secondary to biologic inactive TSH.