**1. Introduction**

Thyroid hormone excess due to any cause is state of thyrotoxicosis, whereas hyperthyroidism is a state of thyroid hormone excess due to hyperfunctioning of thyroid gland. The major causes of thyrotoxicosis include Graves' disease, solitary toxic adenoma, and toxic multinodular goiter. In thyrotoxicosis, free hormone levels are invariably increased. The reverse is not true in that increased free thyroid hormone levels do not always point to thyrotoxicosis. In illness or resistance to thyroid hormones increased free hormone levels are present while the patients are clinically euthyroid or even sometimes hypothyroid. Subclinical thyrotoxicosis is defined as a state in which free thyroid homones (FT4 & FT3) are within normal limit, but serum TSH level is low. The most common cause of subclinical thyrotoxicosis is exogenous administration of thyroid hormone rather than Graves' disease. Patients of subclinical thyrotoxicosis either have no symptoms or have mild non specific symptoms.

The various causes of thyrotoxicosis are listed below in table 1
