Endocrine Abstracts

Introduction: In primary hypothyroidism, thyrotroph hyperplasia can occur due to high thyrotropin releasing hormone (TRH) from loss of negative feedback to hypothalamus. Pituitary thyrotroph hyperplasia can mimic macroadenoma on imaging, but it is reversible with thyroid hormone replacement.

Case: A 34-year-old lady with multiple sclerosis presented atypical headaches and was found to have incidental pituitary macroadenoma measuring 10 x 16 x 10 cm on MRI scan. She had primary hypothyroidism since the age of 15 treated with Levothyroxine 175 mcg/d. She had recently gained 6 kg in body weight over six months and had menorrhagia. Her visual perimetry and range of eye movements were normal. Thyroid profile at presentation found free T3 <1.64 (2.63-5.7 pmol/l), free T4 <5.15 (9.01-19.05 pmol/l), TSH >100 (0.35-4.94 mIU/l) suggesting under-replacement. She was noted to have high TSH for more than 18 months. Her anti Thyroid Peroxidase antibody level >2000 IU/ml. Her prolactin was 177mU/L (109-557), and her cortisol was completely suppressed after overnight 1 mg dexamethasone. Her gonadotrophins and oestradiol level were also in range. Following improved compliance and dietary change with treatment for three months, she had free T3 3.80, free T4 18.10 pmol/l and TSH 1.06 mIU/l. Her weight and menstrual cycles improved, and a follow-up pituitary scan six months later showed a reduction in macroadenoma size, with the pituitary gland returning to normal size and a slightly convex superior profile. This confirms that thyrotroph hyperplasia is reversible with six months of adequate thyroid hormone supplementation.

Conclusion: High TSH had been found to be correlated to pituitary enlargement, and may be a feature at new presentation of primary hypothyroidism as well as those with chronic under-replacement. Reduction in pituitary size is common after thyroxine replacement. Patients should avoid unnecessary surgery and have follow-up scans to confirm resolution instead.