Endocrine Abstracts

Introduction

Isolated TSH (thyroid stimulating hormone) deficiency has been reported in the literature as a rare condition. Only 60 cases have been reported so far since 1960. It has been defined as central hypothyroidism without evidence of any other pituitary hormonal deficiencies or evidence of any structural pituitary abnormality. We report 3 cases of suspected isolated TSH deficiency, 2 of which presented in a year to one endocrine centre.

Cases

Two female patients and one male patient attended our clinic between 2002 and 2004. They all presented with a history of generalised malaise, weight gain, dry skin and in one of the women hair, loss compatible with hypothyroidism . One woman had a low TSH at 0.01mu/l (reference range 0.2-5.0mu/l) with free thyroxine (freeT4) levels in the lowest tertile at 12pmol/l (reference range 9-22 pmol/l). One woman had a low freeT4 level (6 pmol/l) with TSH inappropriately not elevated at 2.2mu/l. The man had low freeT4 (6pmol/l) and TSH inappropriately not elevated at 0.84mu/l. In no case was there evidence of any non-thyroidal illness to account for the abnormal thyroid function tests. In all three cases, the rest of pituitary function was intact to baseline and dynamic testing and thyroid-binding globulin was normal. Magnetic resonance imaging of the pituitary did not reveal any structural abnormality in any of the cases. All patients felt signficantly better with initiation of thyroxine replacement.

Discussion

This series highlights that isolated TSH deficiency may not be as rare as previously thought. The other issue is whether there is a role for thyrotrophin releasing hormone (TRH) testing. We would advocate this to differentiate between hypothalamic and pituitary causes of a low TSH. In all cases of suspected TSH insufficiency, a complete evaluation of pituitary function should be performed.