Thyroid hormones ( THs ), T3 and T4, play an essential role in the development and metabolism of many tissues and organs, and exert profound metabolic effects in adult life, including changes in oxygen consumption, protein, carbohydrate, lipid, and vitamin metabolism.
The effects of thyroid hormones are mediated mainly through T3, which regulates gene expression by binding to the TH receptors (TR)-alpha and -beta.
Thyroid hormones belong to a large superfamily of nuclear hormone receptors which includes steroid hormones, retinoic acid, Vitamin D and peroxisomal proliferator receptors ( PPARs ).
These receptors also bind to enhancer elements in the promoters of target genes, and can regulate both positive and negative transcription.
Recent evidence has characterized some of the molecular mechanisms by which thyroid hormones regulate transcription, as co-repressors and co-activators have been identified, and their effects on histone acetylation examined.
Thyroid hormones also manifest rapid effects that do not require transcription. These can occur via thyroid hormones or other cellular proteins, and typically occur outside the nucleus.
Tendinopathies and tendons tears are not thought to be related to thyroid diseases, but no studies have evaluated in a systematic fashion this association.
In any case, thyroid hormone receptors seem to be ubiquitous.
The relationship between thyroid disorders and shoulder pain has been suspected since the late 1920s, but it has not been systematically investigated.
More recently, however, such association has been more formally hypothesized, and some orthopedic surgeons theorize that thyroid diseases should be linked to idiopathic tendinopathies.
Thyroxine is important for both collagen synthesis and matrix metabolism. Hypothyroidism causes accumulation of glycosaminoglycans ( GAGs ) in the extracellular matrix, which may, in turn, predispose to tendon calcification.
GAGs are involved in the pathogenesis of carpal tunnel syndrome during hypothyroidism. Elevation of glycosaminoglycans, IL6 and TNF has also been reported in exophtalmos in hyperthyroidism.
Tendinopathy can be the presenting complaint in hypothyroidism, and symptomatic relief can be obtained by appropriate management of the primary thyroid deficiency, while calcific tendinopathy has been associated with thyroid dysfunctions. ( Xagena )
Oliva F et al, Muscles Ligaments Tendons J 2013; 3: 201–203