A 63 year old postmenopausal lady with a family history of osteoporosis underwent screening for osteoporosis. Medical history and physical examination was unremarkable except for cystic liver disease. There was no evidence for vitamin D deficiency, hyperparathyroidism or elevation of cholestatic liver enzymes. Her TSH level was slightly enhanced, suggesting subclinical hypothyroidism in presence of TPO antibody positivity. However markers of bone turnover were increased with an alkaline phosphatase 140U/L and P1NP 92ng/ml. As there were no apparent causes for this increased turnover a scintigraphy was performed to rule out Paget disease of bone or other conditions. This revaled no abnormalities except for increased metabolism.

Meanwhile, her thyroid function tests were repeated which revealed a stable TSH of 5,3 mU/L, but also a free T4 just above the upper limit of normal (23.5 pmol/L). Repeated anamnesis did reveal mild complaints of hyperthyroidism and a pulse rate of 80/minute. Assay interference was ruled out.

Therefore it was postulated that the observed increased turnover was possibly due to TSH overproduction. Further biochemical test revealed an enhanced T3 level. Former thyroid function tests performed at her GPs office revealed a normal TSH level several years earlier, making a diagnosis of TSH resistance unlikely, and a secondary hyperthyroidism more likely. An octreotide test, TRH test and MRI were performed. Octreotide test did not affect her TSH levels, however TRH elicited a blunted response of TSH, confirming a TSH producing pituitary adenoma. MRI revealed a pituitary microadenoma.

TSH producing pituitary adenoma are a rare cause of hyperthyroidism, and represent less than 1 % of pituitary adenoma. Signs and symptoms are like in primary hyperthyroidism and can include enhanced bone turnover. Even though this is a rare cause of enhanced bone turnover,hyperthyroidism per se is not and this case confirms the need to rule out secondary causes in subjects who are evaluated for osteoporosis, as underlying conditions are often present and can be addressed.