of $0.3 million, or 4%, in sales of the airway clearance product line in the quarter ended December 31, 2024, compared to the fourth quarter of 2023.
Gross profit in the fourth quarter of 2024 increased $8.4 million, or 15%, to $64.4 million, compared to $56.0 million in the fourth quarter of 2023. Gross margin was 75.2% of revenue, compared to 72.1% of revenue in the fourth quarter of 2023.
Operating expenses in the fourth quarter of 2024 increased $7.6 million, or 17%, to $51.9 million, compared to $44.2 million in the fourth quarter of 2023.
Operating income was $12.5 million in the fourth quarter of 2024, compared to $11.8 million in the fourth quarter of 2023.
Interest income was $0.9 million in each of the fourth quarters of 2024 and 2023.
Interest expense was $0.5 million in the fourth quarter of 2024, compared to $0.9 million in the fourth quarter of 2023.
Income tax expense was $3.3 million in the fourth quarter of 2024, compared to $3.6 million in the fourth quarter of 2023.
Net income in the fourth quarter of 2024 was $9.7 million, or $0.40 per diluted share, compared to $8.2 million, or $0.35 per diluted share, in the fourth quarter of 2023.
Weighted average shares used to compute diluted net income per share were 24.5 million and 23.8 million for the fourth quarters of 2024 and 2023, respectively.
Adjusted EBITDA was $16.2 million in the fourth quarter of 2024, compared to $15.4 million in the fourth quarter of 2023.
Full Year 2024 Financial Results
Total revenue in the full year of 2024 increased $18.6 million, or 7%, to $293.0 million, compared to $274.4 million in the full year of 2023. The increase in total revenue was attributable to an increase of $17.6 million, or 7%, in sales and rentals of the lymphedema product line and an increase of $0.9 million, or 3%, in sales of the airway clearance product line in the full year of 2024, compared to the full year of 2023.
Net income in the full year of 2024 was $17.0 million, or $0.70 per diluted share, compared to $28.5 million, or $1.23 per diluted share, in the full year of 2023.
Weighted average shares used to compute diluted net income per share were 24.1 million and 23.2 million in the full year of 2024 and 2023, respectively.
Adjusted EBITDA was $37.1 million in the full year of 2024, compared to $29.7 million in the full year of 2023.