Does increase in TSH levels affect PTH levels?

Objectives Total thyroidectomy and radioiodine treatment in patients with differentiated thyroid cancer may affect functions of parathyroid glands. Endogenous thyroid hormone stimulation to increase radioiodine uptake in thyroid remnants or metastases is a usual process in patients with differentiated thyroid cancer. Therefore, the aim of this study to evaluate the effect of increased serum thyroid stimulating hormone (TSH) levels on parathormon (PTH) levels.

Methods 50 patients [5 men (%10)] with differentiated thyroid carcinoma were included into the study. TSH, PTH and calcium (total and free) measurements were performed just before the thyroid surgery and the radioiodine ablation treatment. The results were compared by using dependent samples-t test.

Results When patients were euthyroid, TSH, PTH, total calcium and free calcium levels (mean±SD) were 0.88±1.69, 43.85±24.3, 9.41±0.74 and 4.87±0.31, respectively. When patients were hypothyroid, TSH, PTH, total calcium and free calcium levels (mean±SD) were 114.23±66.6, 66.43±36.14, 9.54±0.52 and 4.9±0.24, respectively. While increase in between TSH (p<0.001) and PTH (p<0.001) levels were statistically significant, the change between total (p=0.066) and free calcium (p=0.251) levels were statistically insignificant.

Conclusions According to our results, an increase in TSH levels seems to cause an increase in PTH levels. One possible explanation of this interesting coincidence is that TRH can also cause an increase in PTH levels. Another possible explanation is that increased TSH levels can affect the measurement of PTH levels in immunometric assays. Finally, hypothyroidism can elongate the biological half life of PTH. Further investigations can clarify this interesting issue.