Objective: Primary hypothyroidism can cause disturbances in normal gonadal function. The aim of this study was to investigate the relationship in men between hypogonadism and primary hypothyroidism and the extent to which free and total testosterone levels rose after introduction of replacement thyroxine.
Design: Paired study of patients in a hypothyroid and thyroxine treated state.
Patients: Ten men with primary hypothyroidism.
Measurements: Free and total testosterone, gonadotrophin and prolactin levels before and after thyroxine replacement therapy.
Results: Low free testosterone levels (161 +/- 62 pmol/l) demonstrated at the time the men were hypothyroid rose significantly with the commencement of thyroxine replacement (315 +/- 141 pmol/l; P < 0.001). Gonadotrophin levels were not elevated consistent with hypogonadotrophic hypogonadism. Hyperprolactinaemia, which can occur in primary hypothyroidism and cause hypogonadotrophic hypogonadism, was not present in the majority of these patients. However a reduction in prolactin level was evident with thyroxine replacement and a rise in free testosterone levels.
Conclusion: This suggests an effect of hypothyroidism on gonadotrophin secretion at the level of the hypothalamus-pituitary, either directly or through modulation of prolactin secretion. Low free testosterone may also be a contributing factor to some of the symptoms and signs of hypothyroidism in men.