INFLUENCE OF AGE AND SEX IN MODULATING TSH LEVEL IN PRIMARY HYPOTHYROIDISM

Previous studies addressing the interaction of age and sex with the fu nction of the hypothalamic-pituitary-thyrotrophs axis yielded conflict ing results, due in part to inability to control for the effect of var iable free thyroid hormone levels. We studied the effect of age and se x on TSH levels in patients with severe primary hypothyroidism who hav e essentially undetectable plasma thyroid hormone levels. The TSH leve ls were measured in 116 thyroid cancer patients four weeks after the w ithdrawal of thyroxine therapy in preparation for radioiodine scan/tre atment. All patients had a TSH greater than or equal to 30 mU/L (norma l = 0.2-5) and a free T-4 <6 pmol/L (normal = 10-25). Thirty males and 86 females with a mean age (+/- SD) of 40 +/- 16 (range 6-89 years) w ere studied on up to four hypothyroid episodes, with a total of 191 ep isodes. The TSH level during the first hypothyroid episode correlated significantly with the TSH level during subsequent episodes (first epi sode versus second episode, r = 0.7, P = 0.0001; first versus third ep isode, r = 0.6, P = 0.03). There was a significant negative correlatio n between age and TSH level (r = -0.24, P = 0.0009) that persisted whe n only the first hypothyroid episode was considered (r = -0.23, P = 0. 01), or when only males (r = -0.32, P = 0.02) or only females (r = -0. 23, P = 0.005) were considered. Means of TSH levels in males and femal es were not significantly different (130 versus 114 mU/L, respectively ; P = 0.28). We conclude that age but not sex may modulate the sensiti vity/responsiveness of the hypothalamic-pituitary thyrotroph axis to p rimary hypothyroidism.