Studies of the control of thyroid function in rats: effects of salicylate and related drugs

Rapid alteration in circulating free thyroxine modulates pituitary type II 5' deiodinase and basal thyrotropin secretion in the rat

TSH secretion is decreased by both T4 and T3. This negative feedback control of TSH secretion has been correlated with an increase in pituitary nuclear T3 content, and it is not clear whether T4 exerts its effect directly on the thyrotroph or after its deiodination to T3. However, levels of the pituitary enzyme catalyzing T4 to T3 conversion, 5'D-II, are decreased in the presence of an increased amount of T4. Thus, it is unclear why the thyrotroph would have a mechanism for modulating the production of T3, if T3 is, in fact, the sole bioactive signal providing negative feedback inhibition. To examine this apparent paradox, we administered EMD 21388, a compound which inhibits the binding of T4 to transthyretin resulting in a rapid increase in circulating free T4 levels, to rats pretreated with radiolabeled T4 and T3. We observed increases in pituitary and liver T4 content of greater than 150%, without increases in the respective tissue T3 contents. The EMD 21388-treated rats also exhibited a 25% decrease in pituitary 5'D-II activity (103.8 +/- 15.8 fmol 125I released.mg protein-1.h-1, vs. control, 137.4 +/- 15.9, mean +/- SE), as did rats treated with sodium salicylate, another compound that inhibits T4-TTR binding (100.8 +/- 7.1). TSH levels significantly decreased 2 h after the administration of EMD 21388. These data demonstrate that despite a T4-mediated decrease in pituitary 5'D-II activity, an increase in T4 independently decreases TSH secretion.

The effects of drugs on tests of thyroid function

Many drugs affect tests of thyroid function through alterations in the synthesis, transport and metabolism of thyroid hormones, as well as via influences on thyrotrophin (TSH) synthesis and secretion. Despite effects on circulating thyroid hormone and TSH levels, few drugs result in important changes in clinical thyroid state, but difficulty in interpretation of thyroid function tests often results. Commonly prescribed drugs including anti-convulsants, non-steroidal anti-inflammatory drugs, beta-adrenoceptor antagonists, steroid hormones and heparin may result in abnormal thyroid function tests in the absence of clinical features of thyroid dysfunction. In contrast, lithium and iodine containing drugs, including radiographic contrast agents and amiodarone, may result rarely in overt thyroid disease.

The effect of fenclofenac on thyroid function

The non-steroidal anti-inflammatory agent fenclofenac competitively inhibits the binding of thyroxine(T4) and triiodothyronine (T3) by thyroxine-binding globulin(TBG). Eight male volunteers completed a 4-week study during which they took fenclofenac 600 mg twice daily. The concentration of fenclofenac in serum reached a plateau after 1 week of therapy after which the mean concentration(+/SEM) of the drug in serum was 78.6 o.2 mg/1. During the steady state period on treatment there were reductions of the mean serum concentrations of total T4 to 35% (P less than 0.001), total T3 to 55% (P less than 0.001), free T4 to 69% (P less than 0.001) and free T3 to 90% (NS) of the respective pretreatment values. There were also significant changes in the concentrations of thyrotropin and reverse T3 in serum. After starting treatment with fenclofenac serum concentrations of thyrotrophin fell to a nadir after 2-4 days at which time the mean concentration was 34% (P less than 0.01) of the pretreatment value, whilst reverse T3 values increased to a maximum of 136% (P less than 0.001) of the pretreatment values over 1-2 days. There was subsequently an increase of the thyrotrophin and a reduction in reverse T3 concentrations to normal by 2 weeks of pretreatment. Transient pituitary suppression was also suggested by the response to to thryotrophin-releasing hormone (TRH): 7 days after starting fenclofenac the mean thyrotrophin response was 62% (P less than 9.001) of the pretreatment value. After 4 weeks of fenclofenac the response of TRH had returned to normal. After discontinuing fenclofenac there was a transient increase in the mean concentration of thyrotrophin in serum, to 129% of the pretreatment value (P less than 0.001), with a subsequent return to normal. Four weeks after discontinuing fenclofenac the serum concentrations of thyroid hormones and thyrotrophin were normal.

Salicylate-induced increases in free triiodothyronine in human serum. Evidence of inhibition of triiodothyronine binding to thyroxine-binding globulin and thyroxine-binding prealbumin

Addition of sodium salicylate to human serum at concentrations often obtained during aspirin therapy causes 100-200% increases in free triiodothyronine (T(3)) and free thyroxine (T(4)) as estimated by ultrafiltration. The increase in free T(3) was unexpected since previous data had suggested that salicylate inhibits binding of T(4) only to thyroxine-binding prealbumin (TBPA) and that T(3) is not bound to this protein. Using ultrafiltration techniques, we demonstrated binding of T(3) to TBPA. The affinity constant for T(3)-TBPA binding appears to be slightly greater than that for albumin-T(3) binding. While salicylate inhibits the binding of T(3) (and T(4)) to TBPA, it can be predicted that little change will be observed in the free T(3) (or free T(4)) without inhibition of thyroid hormone binding to thyroxine-binding globulin (TBG). Using a competitive-binding protein displacement technique, it has been shown that sodium salicylate, like diphenylhydantoin (DPH), inhibits the binding of T(3) and T(4) to TBG. The magnitude of the increase in free T(3) and free T(4) induced by salicylates suggests that interference with TBG binding is its major effect. Aspirin was administered orally to two normal subjects in quantities sufficient to obtain serum salicylate levels of 20-25 mg/100 ml. This resulted in a decrease of 20-30% in total serum T(3) and T(4) levels. This decrease in T(4) levels is similar in magnitude to that previously observed in subjects receiving DPH. Unlike what has been observed with DPH treatment, therapeutic salicylate levels are associated with increases of 50-75% in the unbound fraction of both T(3) and T(4) which persist throughout an 8-10 day treatment period.