Beneficial effects of propylthiouracil plus L-thyroxine treatment in a patient with a mutation in MCT8

J Clin Endocrinol Metab. 2008 Jun;93(6):2084-8. doi: 10.1210/jc.2007-2719. Epub 2008 Mar 11.


Context: Mutations of the monocarboxylate transporter 8 (MCT8) gene determine a distinct X-linked phenotype of severe psychomotor retardation and consistently elevated T(3) levels. Lack of MCT8 transport of T(3) in neurons could explain the neurological phenotype.

Objective: Our objective was to determine whether the high T(3) levels could also contribute to some critical features observed in these patients.

Results: A 16-yr-old boy with severe psychomotor retardation and hypotonia was hospitalized for malnutrition (body weight = 25 kg) and delayed puberty. He had tachycardia (104 beats/min), high SHBG level (261 nmol/liter), and elevated serum free T(3) (FT(3)) level (11.3 pmol/liter), without FT(4) and TSH abnormalities. A missense mutation of the MCT8 gene was present. Oral overfeeding was unsuccessful. The therapeutic effect of propylthiouracil (PTU) and then PTU plus levothyroxine (LT(4)) was tested. After PTU (200 mg/d), serum FT(4) was undetectable, FT(3) was reduced (3.1 pmol/liter) with high TSH levels (50.1 mU/liter). Serum SHBG levels were reduced (72 nmol/liter). While PTU prescription was continued, high LT(4) doses (100 microg/d) were needed to normalize serum TSH levels (3.18 mU/liter). At that time, serum FT(4) was normal (16.4 pmol/liter), and FT(3) was slightly high (6.6 pmol/liter). Tachycardia was abated (84 beats/min), weight gain was 3 kg in 1 yr, and SHBG was 102 nmol/liter.

Conclusions: 1) When thyroid hormone production was reduced by PTU, high doses of LT(4) (3.7 microg/kg.d) were needed to normalize serum TSH, confirming that mutation of MCT8 is a cause of resistance to thyroid hormone. 2) High T(3) levels might exhibit some deleterious effects on adipose, hepatic, and cardiac levels. 3) PTU plus LT(4) could be an effective therapy to reduce general adverse features, unfortunately without benefit on the psychomotor retardation.

Publication types

  • Case Reports

MeSH terms

  • Adolescent
  • Antithyroid Agents / administration & dosage
  • Humans
  • Intellectual Disability / complications
  • Intellectual Disability / drug therapy*
  • Intellectual Disability / genetics
  • Male
  • Monocarboxylic Acid Transporters / genetics*
  • Muscle Hypotonia / complications
  • Muscle Hypotonia / drug therapy*
  • Muscle Hypotonia / genetics
  • Mutation, Missense
  • Propylthiouracil / administration & dosage*
  • Puberty, Delayed / complications
  • Puberty, Delayed / drug therapy
  • Puberty, Delayed / genetics
  • Symporters
  • Syndrome
  • Tachycardia / complications
  • Tachycardia / drug therapy
  • Tachycardia / genetics
  • Thyroid Hormone Resistance Syndrome / complications
  • Thyroid Hormone Resistance Syndrome / drug therapy
  • Thyroid Hormone Resistance Syndrome / genetics
  • Thyroid Hormones / blood
  • Thyroxine / administration & dosage*
  • Treatment Outcome


  • Antithyroid Agents
  • Monocarboxylic Acid Transporters
  • SLC16A2 protein, human
  • Symporters
  • Thyroid Hormones
  • Propylthiouracil
  • Thyroxine