Anterior Pituitary Dysfunction
Lactation failure is a very common clinical feature and the lack of prolactin response to administration of thyrotropin releasing hormone (TRH) has been suggested as a sensitive procedure for screening of patients suspected to have SS.[21] Paradoxically, there are also reports of patients with hyperprolactinemia and galactorrhea.[22,23] Gonadotrophic function may be preserved in an occasional patient and there are several reports of patients with SS who maintained regular menstrual cycles and even became pregnant spontaneously.[24–26] Similarly, partial recovery of pituitary function has also been reported.[27] Hyponatremia is the most common electrolyte disturbance occurring in 33–69% of all cases. Several mechanisms are responsible for hyponatremia. Hypothyroidism and glucocorticoid deficiency by decreasing free water clearance independent of vasopressin cause hyponatremia. Syndrome of inappropriate antidiuretic hormone secretion (SIADH) and volume depletion are the other factors leading to hyponatremia.[28–31]
Patients with SS and central hypothyroidism have low Free T3 (ft3) and Free T4 (ft4) with paradoxically normal or mildly elevated serum TSH. They, however, have severely blunted responses of TSH to acute TRH administration and no significant increase in serum TSH or ft4 levels after prolonged TRH infusion. This high level of TSH is due to increased sialylation (a form of glycosylation) which reduces its metabolic clearance leading to increased half-life. However, this glycosylated TSH has reduced biological activity as shown by TSH bioassay based on cAMP generation in a culture system of CHO cells transfected with recombinant human TSH receptor.[32] Abnormal circadian rhythm and increased total TSH secretion due to increased tonic non-pulsatile TSH secretion have also been described in these patients.