Information Types of thyroid disorders Rare Thyroid Disorders Thyroid disorders are medical conditions that occur when the thyroid gland does not function correctly. The most common disorders are dealt with in the BTF information leaflets but there are some other conditions that occur very rarely, sometimes as the result of genetic factors. On this page we highlight some of them. Resistance to Thyroid Hormone Resistance to thyroid hormone is a rare genetic condition where some body tissues do not respond normally to thyroid hormones produced by the thyroid gland. It may be associated with no symptoms or with features of both an overactive and underactive thyroid. To find out more please go to the You and Your Hormones website. MCT8 Deficiency Thyroid Hormone (TH) is crucial for normal development, especially brain development. TH needs to enter cells to be effective; to do this it is transported into the cell by proteins called TH transporters, located in the cells membranes (outer layer). There are a number of different TH transporters but one of these is particularly important for TH transport into brain cells and is called MCT8. The instructions for making the MCT8 transporter are found on the MCT8 gene, which is located on the X chromosome. MCT8 deficiency (also called the Allan Herndon Dudley Syndrome) is caused by an inability to produce a normal MCT8 transporter and is due to a defective MCT8 gene. With a few exceptions, it only affects males. Patients with MCT8 deficiency have severe neurological abnormalities, felt to be due to a lack of thyroid hormone during brain development. Most patients cannot sit independently and cannot walk or talk. They also have high levels of a hormone called T3 in their bloodstream. T3 does not require the MCT8 transporter to get into cells of the muscle, so this tissue is exposed to too high a level of TH. This results in skeletal muscle which is metabolically ‘overactive’ and patients often require a high food intake in order to meet the demands of the metabolically overactive muscle, as well as having low body weight and muscle mass. At present no effective treatment for MCT8 deficiency is available. Effective therapy should at least restore TH signalling in the brain and normalise serum TH levels. Preliminary results indicate that the medication called TRIAC (which is structurally similar to T3) may be effective in MCT8 deficiency. The TRIAC trial in MCT8 patients (https://clinicaltrials.gov/ct2/show/NCT02060474) is designed to determine whether TRIAC is effective in MCT8 deficiency. The primary aim is to evaluate the effect of TRIAC treatment on serum (bloodstream) T3 and other TH levels, with secondary aims being to determine the effect of TRIAC on the toxic effects of the hyperthyroid state in peripheral tissues (such as muscle), and to observe the effects of TRIAC treatment on the neurological features The trial is international, and patients have been recruited to a number of centres worldwide, including Cambridge in the UK. Recruitment for the trial has now been completed. Read Joshua's story Riedel's thyroiditis Riedel's Thyroiditis is a rare form of hypothyroidism caused by a replacement of the normal thyroxine-producing cells in the thyroid by dense fibrosis or scar tissue that invades adjacent structures of the neck. This makes the thyroid gland stone-hard and fixed to adjacent structures. The inflammatory process infiltrates muscles and may affect nerves to the vocal cords causing swallowing difficulties, compression of the windpipe and a weak or husky voice. Surgical treatment is required to relieve tracheal or oesophageal obstruction. Further resources: To find out more about endocrine conditions go to the Society for Endocrinology’s online resource You and Your Hormones.