Thyroid Hormones

• The thyroid regulates its activity based on feedback from the brain.blue brain image by John Sfondilias from Fotolia.com
  
  The thyroid gland coordinates metabolic activity throughout the body by releasing circulating hormones. Levels of two of these, T3 and T4, in the bloodstream provide information on the activity level of the thyroid. The thyroid gland regulates its activity based on feedback from the brain, mainly thyroid-stimulating hormone (TSH) released from the hypothalamus. The blood values of these three proteins (T3, T4, and TSH) taken together determine whether thyroid function is within normal limits. Although these thyroid-related hormones and CK are all involved in cellular metabolism, no direct interaction with CK has been discovered.
  
  

Creatinine Kinase

• Creatinine kinase is involved in energy production in the muscles.muscled runner legs image by jimcox40 from Fotolia.com
  
  CK is found in skeletal muscles as well as the heart and the brain. It does not function in the bloodstream, so high levels of circulating CK (serum CK, or sCK) is generally taken as an indication of trauma to muscle, heart or brain that results in protein leakage into the blood. As early as the 1960s, however, researcher P.D. Griffiths and others associated thyroid disease and abnormal sCK levels. It is now possible to differentiate between CK from different organs and it is most commonly the muscular form that is found at abnormal levels.
  
  

Correlation of sCK with Thyroid Function

• Decreased thyroid function is correlated with increased sCK.graph toys image by Florin Capilnean from Fotolia.com
  
  Several recent studies, including published results in "Endocrine Research" in 2005 and "JK Science" in 2007, noted an inverse correlation between thyroid function and sCK levels: in hypothyroid patients, sCK levels were elevated, while in hyperthyroid patients, sCK levels were lower than normal. After treatment to restore normal thyroid function, sCK levels returned to normal.
  
  

Musculoskeletal Symptoms and sCK Levels

• There is little or no reliable correlation between extent of sCK elevation and severity of symptoms.noise 2 image by iChip from Fotolia.com
  
  Muscle pain or weakness is sometimes associated with hypothyroidism. Since sCK is often associated with muscle damage, researchers have searched for a link between sCK levels and thyroid-associated muscle pain. Two case studies, one in "Hormone Research" in 1999 and another in "Muscle & Nerve" in 2002, illustrate that extremely high sCK levels and extreme musculoskeletal problems can correlate with hypothyroidism, but other studies show little or no correlation. Interestingly, a 1997 study in "Archives of Internal Medicine" showed that three out of four patients with hyperthyroidism experienced muscle cramps during treatment to rapidly reduce thyroid function to normal levels.
  
  

Reasons

• More CK may be released in hypothyroid patients, or the kidneys may simply remove it more slowly.sheep kidney image by PHOTOFLY from Fotolia.com
  
  The reason for the correlation between sCK levels and thyroid function is not clear. Werner and Ingbar's "The Thyroid" suggests muscle fiber degeneration or changes in muscle metabolism due to overall metabolic changes would account for increased release of CK into the bloodstream. A third possibility is that the muscles are not releasing more CK in hypothyroid patients, but that the kidneys remove it from the bloodstream more slowly; this would account for the lack of correlation between sCK levels and musculoskeletal symptoms.