A study published in the Oct 2008 European Journal of Endocrinology shows that patients with pseudohypoparathyroidism type 1a develop hypothyroidism because they're resistant to the effects of a rising TSH.
Pseudohypoparathyroidism is a condition caused by resistance to parathyroid hormone (PTH). This causes them to have a low serum calcium with a high PTH level and associated abnormalities in bone development.