Normally the thyroid gland is controlled directly by the pituitary gland by producing the Thyroid Stimulating Hormone (TSH) (Fig.1). This in turn is controlled by TSH releasing hormone (TRH) from the hypothalamus. In clinical practice only TSH is important because pituitary and hypothalamus are very closely situated diseases affecting one involves the other (i.e. pituitary tumour causing hypothalamic damage). When the thyroid gland produces less thyroid hormone, pituitary senses this and TSH rises to stimulate the thyroid gland. This is called primary hypothyroidism, i.e. the primary problem lies in the thyroid gland itself. In secondary hypothyroidism, the pituitary gland fails to produce normal amount of TSH, so the thyroid gland cannot produce thyroid hormone, in the absence of appropriate stimuli, although by itself it is normal. Opposite is the case when thyroid gland produces more than normal thyroid hormone, i.e. primary hyperthyroidism where thyroid hormone will be high and TSH will be suppressed. In rare occasion of TSH producing pituitary tumour, excess TSH stimulates thyroid gland resulting into thyroid hormone in the circulating blood (secondary hyperthyroidism).

Fig 1- Thyroid Hormone Homeostasis