Hypothalamopituitarythyroid HPT axis

Clinical disorders of thyroid function are known to cause alteration in mood. Classically, patients with hypothyroidism frequently report features similar to depression and, while usually more closely linked to feelings of anxiety, depressive reactions are also sometimes seen in hyperthyroid patients.

In depression, several abnormalities in thyroid function have been described. Figure 2.2 shows the axis, and Table 2.2 lists the abnormalities described. While not entirely consistent, the blunted thyroid-stimulating hormone (TSH) response to thyrotrophin-stimulating hormone (TRH) could be due to hypersecretion of TRH that causes downregulation of pituitary TRH receptors.

Table 2.2 Thyroid axis abnormalities in depression relating to Figure 2.2

A Increased TRH in cerebrospinal fluid

B Reduced TSH response to TRH administration (30%)

C Antithyroid antibodies (9-20%)

D Subclinical hypothyroidism

E Treatment response to T3 or T4 augmentation strategies

Given the suggestions of subclinical hypothyroidism in depression, albeit in a minority, it is natural to ask about the effects of thyroid hormone treatment. A recent review found six randomised, controlled trials, and concluded that thyroid hormone may potentiate both the speed and the efficacy of antidepressant medication (Altshuler et al., 2001). Furthermore, there is also evidence that patients resistant to other treatments may respond to such treatment, although there have been no studies linking pre-treatment, subclinical hy-pothyroidism to this response. There is also some evidence that low tri-iodothyronine (T3) levels are prospectively associated with a higher rate of relapse in the long term (Joffe & Marriott, 2000), and that antithyroid antibodies predict post-partum depression (Harris et al., 1992).

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