Are serum parathyroid hormone (PTH) levels higher or lower in hyperthyroidism?

Serum Parathyroid Hormone Levels in Hyperthyroidism

Serum parathyroid hormone (PTH) levels are typically lower in hyperthyroidism compared to normal levels due to the suppressive effect of thyroid hormones on parathyroid function.

Pathophysiology of PTH in Hyperthyroidism

Hyperthyroidism affects calcium metabolism and parathyroid function through several mechanisms:

• Direct bone effects: Thyroid hormones directly stimulate osteoclastic bone resorption, leading to increased serum calcium levels 1
• Negative feedback: The resulting hypercalcemia suppresses PTH secretion through the calcium-sensing receptors on parathyroid glands 1, 2
• Altered vitamin D metabolism: Hyperthyroidism is associated with increased 24,25-(OH)₂D levels (5.57 ± 3.58 vs 1.73 ± 0.62 ng/ml in controls) 1

Evidence for Suppressed PTH in Hyperthyroidism

Research clearly demonstrates that PTH levels are reduced in untreated hyperthyroidism:

• A study of 22 patients with untreated hyperthyroidism found significantly decreased serum immunoreactive PTH levels (0.23 ± 0.05 vs 0.29 ± 0.05 ngEq/ml in controls, p < 0.05) 1
• These patients also exhibited hypercalcemia (serum Ca: 10.0 ± 0.56 vs 9.0 ± 0.18 mg/dl, p < 0.001) 1
• Bone resorption markers (urinary hydroxyproline, serum alkaline phosphatase, and acid phosphatase) were elevated in all hyperthyroid patients 1

Dynamic Changes in PTH with Treatment

PTH levels respond dynamically to changes in thyroid status:

• After radioiodine treatment for hyperthyroidism, serum PTH concentrations increased in 16 of 17 patients as they became euthyroid or hypothyroid 2
• The increase in PTH was more pronounced in patients who rapidly became hypothyroid (29 ± 15 to 75 ± 29 ng/L) compared to those who gradually became euthyroid (26 ± 16 to 45 ± 24 ng/L) 2
• When patients were treated with L-T4 for hypothyroidism, both serum PTH and TSH decreased 2

Special Considerations: Coexisting Hyperparathyroidism

In rare cases of coexisting primary hyperparathyroidism and hyperthyroidism:

• The hyperthyroid state can mask the expected PTH elevation of primary hyperparathyroidism 3
• Two patients with both conditions showed only slightly increased PTH values despite significant hypercalcemia 3
• As these patients became euthyroid during treatment, PTH levels increased to values clearly suggestive of hyperparathyroidism 3
• This suggests thyroid hormone potentiates the osteoclastic effects of PTH, exacerbating hypercalcemia and producing relative suppression of PTH secretion even from abnormal parathyroid tissue 3

Clinical Implications

Understanding the relationship between hyperthyroidism and PTH has important clinical implications:

• Low or suppressed PTH levels in a hyperthyroid patient are expected and physiologically appropriate
• PTH levels should be interpreted in the context of thyroid status
• When evaluating calcium abnormalities in hyperthyroid patients, the suppressive effect of hyperthyroidism on PTH must be considered
• After treatment of hyperthyroidism, PTH levels typically rise as thyroid hormone levels normalize
• Monitoring both thyroid function and calcium metabolism is important during treatment of hyperthyroidism

Monitoring Recommendations

For patients with hyperthyroidism:

• Check serum calcium levels as part of initial evaluation 4
• Consider checking PTH levels if calcium abnormalities are present 4
• Monitor calcium and PTH during treatment of hyperthyroidism, as normalization of thyroid function may unmask previously suppressed hyperparathyroidism 3